3

Fig. 55. What the North American Continent had become at the end of the Silurian Age. (The modern continent is indicated by dotted lines; the rivers by broken lines.)

during the Lower Silurian Age will answer for a representation of the nature of the events which followed during the Upper Silurian and Devonian ages. Successive extinctions, wrought by the lapse of time, or by violent geological revolutions, followed by successive creations of higher and higher forms, and the annexation of successive belts to the pre-existing land-these constituted the great secular features of the world's history down to the dawn of the period when air-breathing animals were to have birth (Fig. 56). The first period of the Upper Silurian was that during which the Niagara limestone was accumulated-a forma

Fig. 56. A remarkable Silurian Sea-weed (Arthrophycus Harlani). From the Medina Sandstone of the Niagara Group.

tion through which, with others, the Niagara River has cut its way. In another connection I shall have something farther to say in reference to this stupendous piece of Nature's engineering. From the falls of Niagara the outcropping belt of this limestone runs in lines parallel with those just traced. It forms the promontory of Cabot's Head, and the peninsula separating Georgian Bay from Lake Huron. At this point the formation has succumbed to the attacks of the waves, and disappears in its northwestward trend beneath the water of the lake. Cropping out again, it forms the remarkable chain of the Manitoulin Islands, in the northern part of Lake Huron, including Drummond's Island. Beyond St. Mary's River it forms a "point" and a peninsula, the counterparts of Cabot's Head and the peninsula to the south of it. Running westward, and then southwestward, it establishes a continuous barrier to Lake Michigan along the northern and western borders, constituting the rocky ridge which isolates Green Bay and Bay de Noquet from the greater lake. It follows the

shore of Lake Michigan to Chicago, and even to Joliet, when it bends westward and northwestward, and loses itself beneath the accumulations of a later period. The quarries at Lockport, New York, and many others in that vicinity, are located in this important limestone. In the same formation are those at Milwaukee, Waukesha, Chicago, Lamont, and Joliet. The so-called "Athens Marble," so extensively employed in Chicago, is quarried from this formation. It much resembles the famous "Kentucky Marble," from which the beautiful monument and statue to Henry Clay, at Lexington, is built-though the latter comes from the Trenton group, in the Lower Silurian.

The second period was that of the Salina group, which has become famous for the production of salt and gypsum, in the vicinity of Syracuse, New York. Its outcropping belt runs in a line parallel with that of the Niagara limestone throughout its whole course, as far as Milwaukee. I shall hereafter offer some explanation of the circumstances under which salt and gypsum have accumulated to such an enormous extent in certain formations.

The third period was that of the Lower Helderberg group, which is not found to be generally spread out over the country like the other two. In New York it is especially developed in the Helderberg Mountains, where Professor Hall has obtained a rich harvest of organic remains. It was here that he found the type of that magnificent crinoid, which he so beautifully named Mariacrinus, in commemoration of the assistance and sympathy of his accomplished wife in his life-long scientific labors. It thins out and disappears in Western New York. This group is known again in Southern Illinois, where it has been brought to light by the indefatigable and well-directed labors of Professor Worthen; and in Missouri, where it has been illustrated by Swallow and Shumard; and, finally, in Maine,

where it has been studied by Professor C. H. Hitchcock, and Mr. Billings, of Montreal.

Thus closed the Silurian Age. At the east the rocks of this age are marked off from the great mass of overlying Devonian strata by the interposition of a conglomerate— the "Oriskany Sandstone," which signalizes the confusion attendant upon the change of scene. At the West, however, this formation is generally wanting; and we find the limestones of the Corniferous group resting upon those of the Niagara group, except where the Salina rocks intervene. The Corniferous is a most important limestone mass throughout the West. It merges generally into the calcareous portion of the overlying "Hamilton" strata, and forms a landmark in the topography of the country no less than in the series of rocks. In this limestone, quarries are worked from Western New York, in the latitude of Buffalo, through the contiguous peninsula of Canada to Sandusky and Columbus, Ohio, Monroe and Mackinac, Michigan, and multitudes of points in Indiana, Illinois, and Iowa. These limestones, like all others, were accumulated in the bottom of deep and quiet seas. Each successive floor has been the home of moving myriads of sensitive forms. Every layer of rocks has been the cemetery of many generations. Life teemed especially in calcareous and placid waters. Such were those of the Corniferous period; and these limestones are stocked with the relics of ancient dynasties-great and small, powerful and weak, in one wide burial confusedly blent. Nor yet had nature dispensed with the pattern of the trilobites. Encrinites were still in vogue, and orthoceratites, and all the various phases of univalve and bivalve creation. And here-here first dawned upon our planet an animal with a backbone--a mere fish, but yet the basis on which artist Nature has moulded successive models till the form of man shone forth, and the Omniscient was satisfied

to stay his hand. But man was not yet. Ichthyic life seems to have dawned upon our earth in remarkable profuseness. The bones, and plates, and jaws, and teeth of fishes large and small have been cleft from the Corniferous limestone in Canada, Michigan, and Ohio. Our first authentic information of these earliest vertebrates came from Dr. J. S. Newberry-equally distinguished in the service of science and his country--and who has very recently worked up a wonderful collection of Devonian fishes, created mainly by the intelligent industry of a German Methodist minister, Rev. Herman Herzer, while discharging the duties of his ministry at Delaware, Ohio. These ancient fishes were only the avant-couriers of the shoals of sharks, and sturgeons, and garpikes which made a Golgotha of the Old Red Sandstone.

The closing convulsions of this epoch upheaved still higher the growing continent, and depopulated the coral cities of the sea that had just been astir with being. A pause, and another epoch-the Hamilton epoch-followed, a period characterized by its abundance of argillaceous sediments, and by two masses of black bituminous shale— the "Marcellus" at the bottom, and the "Genesee” at the top, with the more calcareous strata between. The absence of the "Marcellus" at the West has dropped the limestones of this group upon the top of the Corniferous limestone, and formed the appearance of but a single mass. This is clearly seen in the extensive quarries upon the islands in the western part of Lake Erie. Indeed, the absence of the "Oriskany" at the West has brought the calcareous portions of four groups of rocks into immediate juxtaposition. These are the Niagara, the Salina, the Corniferous, and the Hamilton. Before these groups were correctly discriminated, the entire mass was known in the West as the "Cliff Limestone." No epoch of the world's history ever