which were soon to be followed by a more or less general disruption. We may generally distinguish, therefore, some calcareous shales constituting the uppermost beds of a group; and, in rare instances, the disturbance proceeded so far before the extinction of the faunas that the uppermost beds have been rendered finely fragmental. To illustrate and confirm these generalizations, I introduce the following table:

It

In this exhibit I take no account of the St. John's Group, since we know so little of its lithological characters. thus appears that the recognized succession of strata in each of the great divisions of Paleozoic time is wonderfully similar in lithological characters. In each great group is a great limestone mass, which stands out conspicuously in the geology of the region underlaid by the group. These limestone masses are prominent landmarks in the progress of the ages. They mark the successive culminations of the geological periods. Each mass outcrops in a protruding belt, sweeping from east to west over a wide extent of country. The oldest is the more northern, and the others follow in regular succession. The Trenton mass sweeps across along the north of Lake Ontario and to Georgian Bay. The Niagara mass lies to the south of Lake Ontario

1. The Lower Silurian Mass (Trenton, chiefly). 2. The Upper Silurian Mass (Niagara, chiefly). 3. The Devonian Mass (Corniferous, chiefly). 4. The Lower Carboniferous Mass. C. Coal-measure Areas.

and the south of Georgian Bay. The Corniferous is north of Lake Erie and beneath Lake Huron. The Mountain limestone is farther toward the centre of the continent, in the Mississippi Valley. The Laramie limestone stretches to the Rocky Mountains. If the reader can fix his imagination on each of these great limestone belts, he has a clew to a mental map of the geology of the country.

In the little map on the preceding page I have endeavored to indicate the locations of the great limestone masses just alluded to (except the Laramie limestone, which is too far west). The horizontal shading shows the trend of the Lower Silurian mass, which, in Ohio and farther west, is not discriminated from the Cincinnati Group. Its prolongation into Wisconsin is covered up with surface sands and clays. The vertical shading indicates the trend of the Upper Silurian mass, which is also lost in Wisconsin. In Ohio it probably exists in a belt encircling the Lower Silurian area, but it has not yet been completely traced out. The oblique shading from right to left denotes the great Devonian mass (corniferous limestone), which has not yet been distinctly traced beyond Lake Michigan. The oblique shading from left to right is the Mountain limestone, or Lower Carboniferous mass, which I have proposed to designate the Mississippi Group, because so extensively developed in the valley of the Mississippi River. Now, if the reader desires to know to what particular formation any proposed limestone quarry belongs, this little map will inform him. The letter C indicates the areas which are underlaid by the coal-measures of the country. In the Northern States these are the uppermost strata of solid rock. Hence all other formations dip toward the nearest coal-measures, and generally pass under them. In other words, all the strata numbered from 1 to 3 dip toward the areas marked C. It follows, also, that Nos. 2 and

3 dip away from No. 1. No. 4 and C are generally nearly horizontal, except in the vicinity of mountain disturbances. Now, by keeping in mind the dips of the several strata,. and tracing each, in conception, underneath those which cover it, the reader will be able to present to his imagination a sort of stereoscopic view of the underground structure of the Northern States.

I have now-leaving out of the account the debatable

Marshall-barely enumerated the epochs of two great periods of the world's organic history, the Silurian and Devonian. Who has considered the measureless intervals which have been so glibly hurried over-the rising and setting suns, the passing tempests, the lonely-budding tree, the sands worried to and fro upon the uncertain beach, the lives of myriads of conscious forms in a long succession of populations, the heaving shore, the rise of continents, the burial of beautiful but senseless ruins beneath acres of sediments from which they shall never be exhumed? Let me commend the sublimity of the theme to the reader's attention.

We are now on the threshold of another great period of the world's history. Graceful tree-ferns are waving in the distance, and giant club-mosses are uttering from their fronds a breezy murmur refreshing to the mind wearied with the contemplation of the uncouth and sombre forms which vegetated in the earlier seas. Looking through the vistas of the future, we behold lazy reptiles reposing upon banks protected by the tangled stems of lepidodendra and calamaria, or floating in the tepid bayous of a tropical jungle. The novelty and interest of the prospect invite us onward, but the vastness of the field bids us pause and refresh ourselves before we venture upon our jottings from the scenes of the Carboniferous Period.