. . .et this period, however long it may have been, was evidently less than the period of activity of these volcanoes.

Fig. 6.

3. Volcanic rocks of an earlier date are also found, sometimes as distinct lavas, though generally as volcanic grits. They occur interstratified with the cretaceous rocks, and with every other formation of the fossiliferous series, showing that, from the earliest times, these rocks have been accumulating as they now are.

The trappean rocks may, in a general classification, be considered as volcanic. It will be shown, hereafter, that they are the lavas of submarine volcanoes. They do not, however, occur in the form of lava currents, but in great tabular masses, generally between stratified rocks, or in the form of dikes. They are also entirely unconnected with cones or craters.

The trappean rocks occur more or less abundantly in all countries. One of the most noted localities of this rock i. . . Read More