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cuss now, as they have no longer anyadherents.

As far as our present experience readies,granite and gneiss seem to belong, pecu-liarly, though not exclusively, to the moreancient rocks: chalk, clay, sand, marl,loam, rock-salt, to the more modern.Greywacke, sandstone, clay-slate, quartz-rock, sienite, porphyry, greenstone, basalt,serpentine, compact feldspar, seem commonto both. In general, the younger rocksexhibit more abraded fragments than theothers, more bituminous and saline matter,more organic remains.

2. ON THE STRUCTURE OF ROCKS.

It has been thought that the primitiverocks were formed by chemical, the flotzby mechanical action; and that, in transi-tion rocks, the two actions were combined.

Of these three opinions, the last, thoughattacked in the Edinburgh 3 Review, is

“ Ed. Review, Vol. ii. p. 343.

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