ronment 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."