653 Professor Sedgwick and R, I. Murchison, Esq., on the 



position, it strongly reminded us of the masses of great scar limestone which 

 rest unconformably on the slate rocks of Cumberland and Westmoreland. 

 From this circumstance, and perhaps still more from some of its fossils, it has 

 been occasionally confounded with the carboniferous limestone. But a portion, 

 at least, of its fossils, more especially its corals, separate it from that limestone ; 

 and the association of its lower beds with calcareous slates passing into the 

 lower slaty system of South Devon, entirely preclude it from being grouped with 

 any part of the true carboniferous system. The coast sections, however, seem 

 to put the question out of all doubt. We have no hesitation in identifying 

 the great limestones of Newton Bushel, Babbacombe Bay, and Torbay. Now 

 the great limestone at the south end of Torbay, after exhibiting a number of 

 contortions, and spreading out into a succession of mural precipices at Berry 

 Head, is finally (at Mudstone sands) bent into a great arch which brings up 

 the lower calcareous slates on which it rests, and causes the southern flap of 

 the great limestone saddle to dip under the slate formation which is expanded 

 towards the south along the shores of Start Bay. The same order of super- 

 position is also indicated atGalmpton Creek on the eastern shore of the Dart, 

 as well as at intermediate spots along the southern boundary of the limestone. 

 (See Section 6.) 



Thus we findj that the great upper limestone of South Devon is fairly inter- 

 polated between two great groups of slate rocks. Under such circumstances 

 we are not surprised to discover that the calcareous beds have a continual 

 tendency to thin out and come to an edge. This phenomenon (as also in 

 North Devon) is best seen in the line of strike. Sometimes, however, we may 

 see the beds thinning otf in the line of dip; so that a mass of limestone will, 

 in a vertical section, look like an acute wedge, driven in between the laminge 

 of the slates. Cases of this kind do not produce any ambiguity, as the longer 

 dimension of these wedges of limestone is always in the direction of the dip. 

 The great limestone of Berry Head thins off in the direction of the Dart, 

 and beyond the river seems to be replaced by several thin calcareous bands^ 

 alternating with the slate. Further west, these calcareous bands disappear : 

 but a similar system, very nearly, if not exactly, on the same line of strike, is 

 revived, and becomes of great thickness, on approaching Plymouth Sound*. 



* Our accompanying section (PI. LI., fig. 8.) shows the lower (or Ashburton) limestone. The 

 beds below this limestone are concealed by an overlying band of culm-measures, as we after- 

 wards learnt from a section by Mr. Austen, and have represented on his authority. The small 

 trough of culm-measures overlying the great upper limestone was observed by us in 18;:56, and is 

 alluded to at its proper place in the text. 



