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, etc, 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 Canademe. 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 X. and S. shores of Lake Superior, and the X. shore of Lake Huron, and constitutes the Green mountain range of eastern Canada and New England, stretching thence northeastward into Newfoundland and south-westward 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 south westward 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 Labra-dorian, 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 Montalban series suggests that they are remaining fragments of great formations once widely spread over an ancient floor of granitic (Laurentian) gneiss; but that these four series mentioned include the whole of the stratified crystalline rocks of North America is by no means certain. How many more formations may have been laid down over this region and subsequently swept away, leaving only isolated fragments, we may never know; but it is probable that a careful study may establish the existence of many besides the four series above enumerated." Notwithstanding the distinction which has been drawn between crystalline and uncrystalline rocks, there is probably to be found somewhere a series of beds marking the passage from these crystalline schists to the uncrystalline sediments of the palaeozoic, although, so far as yet studied, the oldest known strata hitherto referred to the latter are completely uncrystalline, and rest nnconformably upon crystalline eozoic rocks.

There appears to be a close similarity between the latter in widely separated countries, the great series already indicated being recognized with their typical characters in remote parts of the globe.-The palaeozoic rocks have been divided into five great groups, sometimes called systems; but these divisions, as already remarked, are local, and the breaks in stratification and in the succession of organic remains are in some parts filled by beds of passage. As will be seen in the table, there is some difference in the nomenclature of the lower palaeozoic rocks, a portion of the Cambrian of Sedgwick being included by Murchison in the Silurian. In the present account we shall use these terms in the sense in which they were applied by the former. The lower portions of the palaeozoic show no evidence of terrestrial forms of life, their vegetable remains consisting of algae, and their animals of mollusks, corals, and crustaceans. At the summit of the Silurian, however, fishes and amphibians appear, while an abundant land vegetation of acrogens and gymnosperms begins to make its appearance. The palaeozoic rocks are of especial interest to the student of American geology, as they form the surface of the greater portion of the United States east of the Rocky mountains.