Popular Science Monthly/Volume 35/September 1889/Some Modern Aspects of Geology
|SOME MODERN ASPECTS OF GEOLOGY.|
ASSOCIATE PROFESSOR IN THE JOHNS HOPKINS UNIVERSITY.
GEOLOGY has, from the earliest times, claimed the serious attention of mankind, by appealing to two entirely different sides of human character. In the first place, the reverence for the mysterious in nature, which in untutored men amounts to worship, has always been excited by the secrets of the earth; while, in the second place, the cupidity of man has always led him to explore the rocks in quest of the mineral treasures which they contain.
Thus we have at the very outset a theoretical and a practical interest in geology, both of which have played a most important part in the development of the science. From the earliest times and under various guises we can trace their influence side by side, and they are throughout typical of the two objects with which Nature is always studied—as an end in herself or as a means to an end—as science pure or applied.
The ultimate object of geology is to decipher the complete life-history of our planet. The biologist at his microscope succeeds by patient watching in tracing the entire existence of some minute organism. Often the most surprising metamorphoses of form and function are observed, and more than one generation may be necessary to complete the cycle of changes. Through phases far more varied and through conditions infinitely more complex, we may follow the story of "world-life." The globe, like the organism, is developing according to some inherent law of its own; while among its countless fellow-occupants of space it is hardly more than the single insect amid the myriads which compose its swarm.
But the history written in the rocks is long and difficult to read. Here, the record is scanty; there, lost, or, worse still, misleading. Only by the most minute and careful tracing out of every clew can we hope to read aright the glorious tale. A thousand earnest students are collecting observations and comparing their results. Astronomy, physics, chemistry, mineralogy, and biology are all contributing to the sum of what old Mother Earth herself can tell us of her history.
If such a task as this is worthy to arrest the attention and excite the interest of all intelligent men and women, then I may feel justified in speaking of some of the modern aspects of geology.
If we would understand the true significance of the present outlook in geological science, we must take at least a glance at its past history.
Ages before it became a science, geology itself existed. The germs of an interest in the history of the earth are as old as man's own questionings about the origin of himself and his surroundings. In the religions of all ancient peoples are cosmogonies and theories of the world innumerable; and fanciful as these are, they still bear witness to an appreciation of the mysterious in nature amounting even to a worship.
With the advent of Christianity and the acceptance of the Bible, geology became a burning question which has hardly ceased to smolder, even in our day. The Mosaic account of the creation and the true meaning of fossil remains were eagerly discussed by the early Church fathers and by the keenest minds of the Renaissance. Tertullian, Leonardo da Vinci, and Voltaire alike exhausted upon them their sharpest wit and their profoundest wisdom. No assertion could be too absurd to secure a following, provided it accorded with the six creative days. One supposed that the shells imbedded in the rocks on mountain-summits owed their existence to a certain "plastic force" inherent in matter; another imagined them produced by the influence of the sun or stars. Still others were so blasphemous in their mad defense of Scripture as to assert that fossils were only the waste debris formed in earlier and unsuccessful attempts of the Deity to create a world. And, lastly, Voltaire, in bitter irony, maintained that in his opinion the fossils of the mountains were merely shells dropped from the pilgrims' hats as they journeyed homeward from the Holy Land! The decrees of religious dogma as to what interpretation was to be placed upon facts which the rocks disclosed, were as stern and implacable as those placed by the Church on Galileo; but still more stern and implacable were the facts themselves. For centuries the fierce war raged on one battle-field after another, and from each, Dogma sullenly retired, leaving the victory to Truth. This fascinating phase of the history of geology has been made the subject of a series of recent papers by President Andrew White. It does not, however, concern us further than to show that, although such violent opposition certainly retarded the early and free development of geology, it was nevertheless not unfavorable to its ultimate success. The wide-spread partisanship excited by theological discussions only disseminated a broader knowledge of the subject, and hence a greater interest in it, so soon as the hindrances to its cultivation were finally removed.
But it is to neither religious persecution nor to religious zeal that we owe our modern science of geology. Dogma and discussion might have been extended indefinitely without approaching one whit nearer to the truth. Observation, not theory, was the one thing needful. While the doctors were deciding whether or not shells could have been strewn over mountain-tops by the waves of Noah's deluge, the "practical men" of the earth were busy in exploring its crust for hidden wealth. Some accurate means of comparing and classifying the strata was to them a matter of necessity, and it need not surprise us to find that the first real geologists were not professors, but "practical" miners; that the earliest germination of a truly scientific study of the earth was not in the university, but in the technical school.
At that remarkable period, about one hundred years ago, when not merely the sciences, but Science herself in the modern sense, sprang into life, geology was doubly prepared to receive the benefit of the great awakening. As she gradually developed from a creed into a science, there was twofold interest in her welfare: the first, theoretical, or, as we may more properly say, theological, since it amounted to a religious fanaticism; the second, practical, and brought about by the growth of mining industries and the search for wealth.
During the past century of geological activity the objective points of these two ideas have been in succession more or less cultivated. Among the theologians the question at issue related to the fossils; among the miners, on the other hand, to the rocks.
Originating, as the systematic study of the earth's crust did, in the mining schools, it is not strange that the latter first received the serious attention of scientific men. The rocks were the earliest objects of investigation, and petrography, or the science of rocks, was, naturally enough, the starting-point in geology. But as a science, petrography was, at the outset, a failure, though not on account of any lack of appreciation or patience on the part of its cultivators. Mineralogy throve, but no means could be discovered of applying her methods to the finer-grained rocks, and so the interest in petrography necessarily declined. After repeated trials, resulting only in disappointment, the students of rocks followed the example of the theologians; and, in lieu of observations and facts, produced only the useless and often virulent polemics of the Neptunist and Vulcanist.
Again, there was a reaction against such waste of energy. Geologists, wearied by more barren controversy, turned eagerly to some new field where observation should be less difficult. They had opened the great book of Nature and had first tried to read the text; but the hieroglyphics were obscure, and the clew could not be found. Is it strange, therefore, that they should have gladly left this hard and unintelligible writing for the picture-book which Nature spread before them in the fossils? Here at least was something tangible. None now doubted that these fossils had once been living organisms which could be understood by careful comparison with living forms.
It was through the study of fossil organisms—or paleontology—that geology first accomplished its true aim, viz., the deciphering of a portion of the earth's history by observed facts. We can hardly wonder that a field so fruitful should, since the beginning of our century, have been cultivated to the exclusion of almost every other. But paleontology is essentially a biological, not a geological science. Its service to the sum of human knowledge can scarcely be overestimated, for it has done much in establishing the greatest generalization of this or perhaps of any century—the doctrine of evolution. Nevertheless, its contributions must ever be to the history of life on the globe, rather than to the history of the life of the globe.
So strong has been the growth of the organic side of our science that a popular idea still prevails that there is no geology aside from stratigraphy and the fossil-bearing rocks. The paleontological school is still in the ascendant, but it is no longer without a vigorous rival.
Within recent years there seems to have been infused into almost every domain of physical science a fresh life. Through gradually acquired generalizations higher points of view have been reached; old notions have been discarded for newer and broader ones. Prof. Langley tells us of the "new astronomy"; the doctrine of the conservation of energy has given us a new physics; evolution, a new biology; and the study of carbon compounds, a new chemistry. So, too, the application of the microscope to the study of rocks has given us a new geology.
The recent development in the science of the earth consists of the return to the work begun by its earliest pioneers. The old Petrographers were right. If we would know the life-history of our planet, we must learn the origin, structural relations, and composition of our rocks. We must discover the forces— chemical and physical—which work in and upon them, and we must see how they work.
As I have already said, the early geologists had full faith in the importance of their labors, but they were forced to abandon them by a lack of methods and appliances suitable to cope with the difficulties presented. To-day this importance is not diminished, but rather increased, by what has been accomplished along other lines. If we can renew the attack upon the old questions with improved weapons, the rewards of victory are as promising as ever. It is believed that such weapons are now in our hands, and the hope of success is almost daily attracting fresh and earnest workers to the ranks from every land.
The first and strongest impetus to a renewed study of the rocks themselves was given by the successful application of the microscope to this end; but this most valuable acquisition has by no means remained alone in the rapid growth of modern petrography. Other appliances, scarcely less useful in rock-study, followed quickly in its wake. Microchemical analysis, the separating funnel, and, most of all, the furnace, in which has been accomplished the perfect synthesis of many rocks, have all contributed, along with the microscope, to make the methods of petrography not inferior in delicacy and accuracy to those of any other science.
The greatest difficulty with which the older geologists had to contend, in their studies of the rocks, was their inability to identify the constituent minerals which composed them. Their disappointment and vexation are still curiously recorded in some of our oldest rock-names, like "dolerite," deceptive; and "aphanite," not apparent or distinguishable. With the successful application of the microscope to rock-study, this difficulty at once disappeared, and at the same time new and unexpected problems of the greatest interest unfolded themselves in quick succession.
In the light of all that had been done with the aid of the microscope in the organic sciences, it may at first seem strange that its application to geology was so long delayed. This was due to the imaginary difficulties in preparing transparent rock sections, and to the fact that rock powders had been examined microscopically at an early date with absolutely no result.
In spite of certain sporadic efforts in this direction, it was not until the year 1858 that the clew to the solution of the difficulty was hit upon by Henry Clifton Serby, a wealthy manufacturer of Sheffield, England, who as a pastime succeeded in making transparent rock-sections. These he examined with the microscope with good results, but the matter would hardly have received serious attention by scientific men had he not, almost by accident, transplanted his idea to Germany. In this congenial soil it readily took root and flourished like a vigorous tree, bearing rich fruit and sending its seeds into every land upon the earth where knowledge is sought for.
At first progress was necessarily slow, mistakes were frequent, and a general interest in the subject was almost lacking. But as one point after another was gained, and as a deeper insight into the problems presented was secured, the number of workers steadily increased. The patient labors of such pioneers as Zirkel, Vogelsang, and Rosenbusch can never be forgotten by those who can now avail themselves of their years of toil in a few months.
Interesting and surprising results were secured at the outset by the new science, but they were mineralogical rather than geological in their bearing. It is only now, after thirty years of preparation, that the time is fully ripe for the application of the new petrography to some of the deepest questions of theoretical geology. This it is which affords almost the only hopeful means of dealing with the records of the crystalline strata of the earth, which undoubtedly contain the longest, as they do by far the darkest, chapter of its history. What paleontology has already done and is still doing for the more superficial strata in which organic remains are preserved, the microscope must do for the crystalline rocks, whether volcanic, plutonic, or metamorphic. These contain their own life-histories, written in characters which need only to be carefully studied in order to be properly interpreted.
The purely mineralogical services of the microscope need not here concern us, but it may be pertinent to inquire. What specific classes of facts has this instrument disclosed and what new ideas has it suggested that entitle it to so high a consideration by those who are interested with the broader problems of the earth's history? To this inquiry we may answer:
1. The microscope has shed light into darkness; and, by its promise of results, has stimulated an enthusiastic cultivation of a most important but hitherto neglected field.
2. It has shown us that the internal structure of the commonest pebble is not less admirable, delicate, and exquisitely beautiful than that of a living organism.
3. It has already thrown much light upon the origin of many of the crystalline rocks—both massive and schists—by allowing us to judge of the conditions under which they must have been formed.
4. Most wonderful of all, it has taught us that the components of the "everlasting hills" are not mere masses of dull, unchangeable, inert matter, but that, in so far as constant change of form and composition to accord with altered conditions is a sign of life, they live.
Any single one of the four points which I have here enumerated is enough to assure a lively interest in modern petrography, not merely on the part of geologists, but on the part of all intelligent persons who love to study the "wonderful wisdom and power of God as shown in his works." Together they promise far more for the future than has been fulfilled in the past.
We can not pause long enough to consider each of these four points in succession, but it will be worth our while to glance for a few moments at the last.
It is a question how far the popularly received distinction between dead and living matter can be made amenable to strict definition as long as we know so little of what the so-called "life force" is. As far as we can judge of the phenomena presented by the organic and mineral worlds, they differ rather in degree than in kind. This seems like a bold statement, and I am fully aware that it would be totally unwarranted except for the recent disclosures of the microscope in geology.
The chemistry of life is the chemistry of carbon; the chemistry of the rocks is the chemistry of silicon. Both are closely allied elements, with the property of forming extremely complex compounds, which become more or less unstable with a variation of external conditions. We are accustomed to regard unceasing change as a sign of life, and to look upon the rocks as unchanging, and therefore dead. But the microscope shows that this is a false conception. Not only do the component minerals assume a form as directly inherent in their nature as that of a plant; but, if the surrounding conditions become unfavorable, they change to other forms, and leave written in the rocks the records of their often complicated histories. The only difference seems to be in the relative slowness of the action. I say "seems to be," because I am by no means convinced of the absolute identity of the two processes.
In his recent annual address, the well-known President of the Geological Society of London, Prof. John W. Judd, has attempted to throw aside entirely the distinction between crystallized and living matter, and to bring the phenomena of change observed by the microscopist in rocks within the limits of such definitions of life as those of Lewes and Spencer. While we may be unwilling to follow him to this extent, we can but confess that the analogy to vital terms and processes recently used with so much power by Prof. Drummond in quite a different sphere is also capable of a valuable application in illustrating some of the modern aspects of geology. We may speak of the embryology of a mineral, of its histology, morphology, physiology, vitality, and suitability to its environment, designating by these terms phenomena which are at least analogous to those which they represent in biology.
We encounter, in thin sections of both, volcanic and metamorphic rocks, microscopic crystals arrested in every stage of their growth, and it is not true that these earlier forms are mere epitomes of the perfected individual. We have the fundamental globulite and the complicated and fantastic "growth-forms," which are as different from the finished crystal as is the larva from the butterfly. Thus, to one familiar with such facts as these, there can be no confusion in speaking of the "embryology of a crystal." We think with wonder of the marvelous vitality of seeds which sprouted after three thousand years spent in Egyptian pyramids, and yet the "vitality" of a crystal is such that it will continue its growth under favorable conditions after any number of thousands of years of interruption.
There is, however, nothing among the recent disclosures of the microscope in regard to rocks so surprising as their delicate adjustment to their environment. We are accustomed to look upon the masses of our mountains as the very type of what is stationary and eternal; but in reality they are vast chemical laboratories full of activity and constant change. With every alteration of external conditions or environment, what was a state of stable equilibrium for atoms or molecules ceases to be so. Old unions are ever being broken down and new ones formed. Life in our planet, like life in ourselves, rests fundamentally on chemical action. The vital fluid circulates unceasingly through the arteries of the oceans and the currents of the air; it penetrates the rocks through the finest fissures and invisible cracks, as the human blood penetrates the tissues between artery and vein, producing, with the help of heat and pressure, like changes in the histology of the globe. The recurrence, after a long interval, of the same set of conditions in the same rock-mass, may bring about the unending cycle—analogous to succeeding generations—which Hutton, the earliest of the Scotch geologists, recognized a hundred years ago.
Such processes as these, which properly represent the physiology of our earth's crust, have long been suspected, but their exact nature and details are only now being gradually disclosed by microscopical studies of the rocks.
Suppose, for instance, that a lava-stream bursts from the side of some volcano. As it flows onward, quickly solidifying and crystallizing under circumstances of intense heat, chemical compounds are produced which accord with such conditions, but perhaps not with those ordinarily obtaining at the earth's surface. If this is the case, the hardened lava will be in a chemically unstable state, and will tend in turn to adapt itself to its new surroundings by chemical change.
Countless examples of this adaptability of rocks to their environment are familiar to every geologist who has availed himself of the newest and most potent aid in his profession. There is nothing hypothetical about them, for the minerals have written their own "life-histories" in characters which can not be misread. They throw a flood of light upon many types of rocks whose origin and nature have heretofore remained an unsolved riddle; and they open up a vista of possibilities to the future explorer whose length and whose attractiveness can hardly be exaggerated.
Let me quote, in closing this brief survey of a new field in geology, a single passage from Prof. Judd:
"In the profound laboratories of our earth's crust," he says, "slow physical and chemical operations, resulting from the interaction between the crystal, with its wonderful molecular structure, and the external agencies which environ it, have given rise to new structures, too minute, it may be, to be traced by our microscopes, but capable of so playing with the light-waves as to startle us with new beauties, and to add another to
'The fairy tales of science,
And the long results of time.'
"Yes! minerals have a life-history, one which is in part determined by their original constitution, and in part by the long series of slowly varying conditions to which they have since been subjected. . . . In spite of the limitations placed upon us by our brief existence on the globe, it is ours to follow, in all its complicated sequence this procession of events; to discover the delicate organization in which they originate; to determine the varied conditions by which they have been controlled; and to assign to each of them the part which it has played in the wonderful history of our globe during the countless ages of the past.
"Mineralogy has been justly styled the alphabet of petrology; but if the orthography and etymology of the language of rocks lie in the province of the mineralogist, its syntax and prosody belong to the realm of the geologist. In that language, of which the letters are minerals and the words are rock-types, I am persuaded that there is written for us the whole story of terrestrial evolution."
- Portion of an address delivered at the commencement exercises of the Worcester Polytechnic Institute, June, 1888.