The reason of this is obvious.

There are many places--to take a single example--where one may see the the Silurian rocks, the Devonian, and the Carboniferous rocks succeeding one another regularly, and in their proper order.

This is because the particular region where this occurs was always submerged beneath the sea while these formations were being deposited.

There are, however, many more localities in which one would find the Carboniferous rocks resting unconformably upon the Silurians without the intervention of any strata which could be referred to the Devonian period.

This might arise from one of two causes: 1.