2. GEOLOGICAL STRUCTURE OF THE TOADSTONE DISTRICT.--As the volcanic rocks of Derbyshire lie among the Carboniferous Limestones of a broad anticlinal dome, they are only exposed where these limestones have been sufficiently denuded, and as the base of the limestones is nowhere laid bare, the lowest parts of the volcanic series may be concealed. Over the tract where the toadstones can be examined they appear as bands regularly intercalated with the limestones, but varying in thickness in the course of their outcrops. As they are prone to decay, they usually form smooth grassy slopes between the limestone scarps, though isolated blocks of the dull brown igneous rocks may often be seen protruding from the surface. Now and then a harder bed of toadstone caps a hill, and thus forms a prominent feature in the landscape, but as a rule these igneous bands play no distinguishing part in the scenery, and are indeed less conspicuous than the white escarpments of limestone which overlie them. It was the opinion of the older geologists that three distinct platforms of toadstone extend without break throughout the district, and subdivide the limestones into four portions. But this opinion does not seem to have been based on good evidence either of sequence or of continuity. Various facts were brought forward by the officers of the Geological Survey to show that the supposed persistence of the three platforms of toadstone did not really exist, but that these sheets of igneous material are found at different spots on very different horizons, and are of limited horizontal range.[31] So far as my own limited observations go, they entirely corroborate this view. There can be little doubt, I think, that the identity of certain outcrops of toadstone has been assumed, and the assumption has been carried throughout the district. The truth is that the number of successive platforms on which igneous materials appear will never be satisfactorily determined until the stratigraphy of the Derbyshire Carboniferous Limestone is worked out in detail. When the successive members of this great calcareous formation have been identified by lithological and palæontological characters over the district, it will be easy to allocate each outcrop of toadstone to its true geological horizon. When this labour has been completed, it will probably be found that instead of three, there have been many discharges of volcanic material during the deposition of the limestone series; that these have proceeded from numerous small vents, and that they are all of comparatively restricted horizontal extent. Such a detailed examination will also determine how far the toadstones include veritable sills, and on what horizons these intrusive sheets have been injected. [Footnote 31: _Geol. Surv. Mem. on North Derbyshire_, by Messrs. Green and Strahan (1887), p. 104.]