The volcanic group of Derbyshire thus stands out entirely isolated. Lying in the Carboniferous Limestone, where that formation is typically developed, it presents an admirable example of a thoroughly marine phase of volcanic action (Map I.). One of the most prominent features in the geology of the centre of England is the broad anticlinal fold which brings up the lower portion of the Carboniferous system to form the long ridge of the Pennine chain that runs from Yorkshire to the Midland plain, and separates the eastern from the western coal-fields. This fold widens southwards until not only the Millstone Grit and Yoredale rocks, but the underlying Mountain Limestone is laid bare. A broad limestone district is thus exposed in the very heart of the country, ranging as a green fertile undulating tableland, deeply cut by winding valleys, which expose admirable sections of the strata, but nowhere reach the base of the system. The total visible depth of the limestone series is computed to be about 1500 feet; the Yoredale shales and limestones may be 500 feet more; so that the calcareous formations in which the volcanic phenomena are exhibited reach a thickness of at least 2000 feet. It is not yet definitely known through what vertical extent of this thickness of sedimentary material the volcanic platforms extend, but where most fully developed they perhaps range through 1000 feet, lying chiefly in the Carboniferous Limestone, but apparently in at least one locality extending up into the lower division of the Yoredale group. The area within which they can be studied corresponds nearly with that in which the limestone forms the surface of the country, or a district measuring about 20 miles from north to south, with an extreme breadth of 10 miles in an east and west direction. A special historical interest belongs to the Derbyshire "toadstones."[18] They furnished Whitehurst with material for his speculations, and were believed by him to be as truly igneous rocks as the lava which flows from Hecla, Vesuvius or Etna. But he thought that they had been introduced among the strata and "did not overflow the surface of the earth, according to the usual operations of volcanoes."[19] [Footnote 18: This word has by some writers been supposed to be corrupted from _tod-stein_, dead-stone, in allusion to the dying out of the lead veins there; by others the name has been thought to be derived from the peculiar green speckled aspect of much of the rock, resembling the back of a toad.] [Footnote 19: _An Enquiry into the Original State and Formation of the Earth_, 1778, Appendix, pp. 149, _et seq._]