Probably the true answer to this, though we cannot now give adequate proof of it, is that the disintegration of granitic and other igneous rocks, to which, on what seem good grounds, we have already ascribed the origin of gneiss and mica schist, has been the prolific source of all these sedimentary strata. Analysis of the principal rocks of the slaty systems does certainly not contradict this view; which neither those who admit with Leibnitz the first solid covering of the globe to have been a mass of rocks cooled from fusion, or, with Lyell, that strata added above, are melted and reabsorbed into granite below, have any reason to deny
Moreover, we see daily, on the slopes and at the foot of hills composed of trap rocks, considerable quantities of loosely aggregated sands, which to all appearance, if agitated in water, might be indistinguishable from various secondary or Silurian sandstones. The abundant detritus which surround the basaltic hills of Rowley, the sienites of Mount Sorrel, and the granites of Arran, are in this respect very worthy of attention, and may suggest to those who have the opportunity a train of valuable research, which might elucidate many points now obscure in the history of the disintegrated materials of igneous rocks.
Devonian System.
The composition of this group of strata is formed upon one general model in all the parts of Britain north of the Severn; but we may distinguish two varieties in this plan. First, the series of the Grampians, Lammermuirs, and Cumbrian region, may be thus stated in general terms:—
