other southern regions more remote from the northern source of the sediments the rocks of the age are represented by the "Mountain Limestone." [See Appendix, Note IV.]

ocean.

I am here led to direct the reader's attention to an important law which has governed the distribution of sediments in all the periods of American paleozoic history. The continent, it will be remembered, was always toward the north. Soundings in the North Atlantic indicate that the actual foundations of the continent extend northeastward beneath the water far beyond the limits of the existing land. Far back in the antiquity of our continent the Labrador branch possessed an extent which no longer appears. It projected itself in that direction almost to midIt has been eaten up by the waves of the Atlantic. The bones of the continent lie scattered along from the "Grand Banks" to Maine. Newfoundland, Cape Breton, Nova Scotia, and the numberless islands and peninsulas of the northeast coast, are the remnants of the meal which old Ocean has made of the right wing of America. Out of the wasted continent of paleozoic times the agencies of Nature have built up the substructures of the Northern United States. All the strata to which I have referred were formed of the ruins of rocks that had long before been dry land. Thus the materials came from the northeast. And thus it happens that every formation is coarser in that direction, and finer toward the centre of the continent. Thus even the age which witnessed the accumulation of pebbles or sand at the East, witnessed the deposition of a fine calcareous mud in the deeper, quieter waters which rested over the Mississippi Valley (compare Fig, 15). Another thought introduces itself into the company of this one. It is the law of the secular recurrence of iden tical lithological conditions. This law attracted my atten

tion many years ago, but I believe no one has distinctly enunciated it except that admirable geologist, Dr. Dawson, of Montreal. Geological time has been marked off into Ages, Periods, and Epochs by physical revolutions. These were universal for the Ages, but more local for the subordinate divisions of time. The commencement of every interval of time was characterized, to some extent, by disruptions, upheavals, violence, emission of heat and vapors from beneath the crust, violent dashing of waters against coastbarriers, destructive ocean tides and streams, and the more or less complete extinction of living beings. Simultaneously, therefore, with the disappearance of a fauna from the earth, the ocean's bottom was overstrewn with the coarse débris of a geological revolution. As the shaken crust subsided to a more quiet position, only the finer sediments were transported to great distances from the shores. Lastly, when peace and stability were again restored, the vast expanse of the ocean, as it floated over the area of North America, was a calm and clear lagoon, in which lived and labored those lime-loving animals which incase themselves in shells, found coral structures, and eliminate from the water the materials of limestone strata. There is, consequently, for each period of the world's history a definite succession of strata as to kind. These may be designated Coarse-fragmental, Fine-fragmental, and Calcareous. The Coarse-fragmental we style conglomerates, and their position is at the bottom of a group of strata. The Fine-fragmental vary from sandstones to shales, and they rest upon the conglomerates. The Calcareous constitute the limestones which answer to the culmination of a geological interval, and rest near the top of the group. The life of each interval attained its full expansion during the Calcareous epoch. Toward the close of this epoch the waters of the sea began again to be turbid, from the premonitory jarrings

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:

In this exhibit I take no account of the St. John's Group, since we know so little of its lithological characters. It 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

Fig. 58. Trends of the great Limestone Masses.

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