The contorted strata of slate and greywacke rock must have been formed at a period vastly anterior to that in which the lake of the upper valley managed to force an outlet through the enclosing barriers.
The bands of massive grit and coarse greywacke, for example, break up into larger blocks and from their greater hardness are apt to project above the general surface of the other softer rocks.
In that belt they consist mostly of greywacke, grit, shale and other sedimentary rocks, but in the southwest of Ayrshire they include some thick lenticular bands of limestone.
This fossil-bearing set of strata he first styled the "fossiliferous greywacke series," changing it in 1835 to "Silurian system."
On the northern side, which sinks on the whole gently towards the Lahn, the greywacke formation attains a considerable development.
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