Page:The American Cyclopædia (1879) Volume VII.djvu/707

of sea and land, it follows that there are breaks in the succession of the rocks, which are often marked by a want of conformity in the arrangement of the successive formations. The sea retires from an uplifted continent, the strata become more or less disturbed, and perhaps in the course of ages partially broken down and swept away. When a new movement of the earth's crust brings this region once more beneath the sea, a new series of beds resting horizontally upon the older formation is deposited, and we have evidence, both from the relations of the strata and from the changes in the organic remains, of a break in the succession. Yet it is clear that elsewhere in the region occupied by the sea during this interval would be deposited sediments which fill up the interval. The process of deposition of sediments in the sea has never been interrupted, though the area of deposition has changed, and all breaks in the succession are local and accidental interruptions. Our divisions into systems and groups have been based in great part upon these interruptions, corresponding to omitted leaves in the succession, which the progress of investigation is now gradually supplying, so that the record when completed will show no breaks and no interruption either in the deposition of strata or in the succession of the forms of life. The disturbances or cataclysms which in the theories of the older school of geologists were looked upon as universal are really local, and are dependent upon the disturbances due to slow movements and the transfer of the process of sedimentation to other regions. But it is precisely where these breaks have been noticed that geologists have established horizons or lines of demarcation upon which the systems of classification have been built. From time to time we find out the formations which in other regions correspond to these interruptions, and serve to show the transition from one of the periods to another. These limits between hitherto separated formations are designated beds of passage. It is proposed to give a brief sketch of the successive geological groups enumerated in the preceding table, commencing with the lowest or eozoic period, and to notice the principal facts in their history, more especially as seen in North America.—The rocks which we have called eozoic include the crystalline strata, which are regarded in the present state of our knowledge as forming four great groups marked by lithological differences. At the base we have placed the Laurentian, which consists in great part of granitoid gneiss, in which, but for the interposed strata of quartzite, crystalline limestone, &c., there would in many parts be found small evidence of its stratified origin. This ancient group is what is called in Scandinavia the primitive gneiss, and corresponds to the fundamental granite which is often spoken of as underlying all other rocks. It is the oldest series of rocks known, and in North America forms a large part of the Laurentides, the Adirondacks, the Highlands of the Hudson, and their continuation southward. The thickness of this great series is unknown, but Sir William Logan has estimated that at least 20,000 ft. of strata belonging to it are exposed on the Ottawa river. It there includes three great limestone formations, which are associated with iron ore, plumbago, and phosphate of lime, and contain the remains of a foraminiferous organism to which Dawson has given the name of eozoon Canadense. To the Laurentian succeeds what has been named the Huronian, a group of crystalline rocks much more schistose than the Laurentian, and consisting of imperfect gneisses, with micaceous, chloritic, and talcose schists, and beds of hornblende and serpentine rocks, associated with argillites and magnesian limestones. This series is widely spread along both the N. and S. shores of Lake Superior, and the N. shore of Lake Huron, and constitutes the Green mountain range of eastern Canada and New England, stretching thence northeastward into Newfoundland and southwestward along the Appalachians. Rocks apparently belonging to this series fringe portions of the E. coast of New England, and are seen in a wider development in the coast range of southern New Brunswick. In some parts of the Lake Superior region the Huronian rocks are found to rest unconformably upon the Laurentian, and to be made up in part of its ruins, thus indicating a break between the two series. The third great group noticed in our table is that of the White mountains, or, as it may be called, the Montalban series. It consists in great part of gneisses, which, however, are lithologically dissimilar from those of the Laurentian, and are associated with large bodies of highly micaceous schists, abounding in kyanite, staurolite, andalusite, and garnet. This series of rocks is traced from the White mountains northeastward across the state of Maine and southwestward throughout the Appalachians. The facts, so far as known, seem to show that it is newer than the Huronian, resting unconformably upon it, and in some places probably upon the Laurentian in the absence of the former. The fourth group is what has been called the Norian or Labradorian, which consists in great part of granitoid or gneissoid varieties of the rock called norite, consisting chiefly of Labrador feldspar. With this are associated gneisses, quartzites, and crystalline limestones not unlike those of the Laurentian. This series in various parts of Canada and in northern New York appears to rest unconformably on the Laurentian, and was hence called by Sir William Logan the upper Laurentian; but according to recent observations by Hitchcock, it occurs in New Hampshire, apparently overlying the White mountain series. Dr. Sterry Hunt, who is the author of this attempt to group and classify the eozoic rocks, remarks: “The distribution of the crystalline rocks of the Norian, Huronian, and