Volcanic rocks, locally called "Toadstone," are represented in the limestones by intrusive sills and flows of dolerite and by necks of agglomerate, notably near Tideswell, Millersdale and Matlock.
A few hornblendeschists are metamorphosed gabbros; others have developed from dikes or sills of lamprophyre.
The chief portion of this rig is the derrick, Oil which consists of four strong uprights or legs held in Derrick position by ties and braces, and resting on strong wooden sills, which are preferred, as a foundation, to masonry.
The samson-post, which supports the walking beam, and the jack-posts, are dove-tailed and keyed into the sills.
The depth of water on the dock sills varies from 172 ft.