1851.] SEDGWICK SLATE ROCKS OF DEVON AND CORNWALL. 15 



1 . It might be supposed probable that the rocks on both sides of 

 the granitic axis, between the Land's End and St. Stephen's Moor, 

 near St. Austell, belonged to an older system than the corresponding 

 rocks in the other parts of the county : and I have already noticed 

 the difficulty presented by the sections along a line drawn from St. 

 Austell Bay to the north coast. 1 do not, however, believe that this 

 hypothesis is at all probable ; but it ought to be tested by a severe 

 examination of all the sections between Truro and Falmouth, and in 

 all the southern parts of the county, where the condition of the rocks 

 offers any chance of discovering fossils. It had been stated that Mr. 

 Peach found Graptolites at Black Head, a few miles south of St. 

 Austell. On examining the specimens of the supposed GraptoliteSy 

 in the museums of Penzance and Truro, Prof. M'Coy found that they 

 had been wrongly named. They belong to a new species of Cladocho- 

 nns, which is a Coral of a Carboniferous genus * : and at Pridmouth, 

 near Menabilly, on the opposite side of St. Austell Bay, the same 

 species of Cladochonus is found amongst many undoubted Devonian 

 species which we collected on the spot. 



2. The second and more probable hypothesis is, that a great y«w?^, 

 with an upcast towards the south, has brought the fossiliferous group 

 above described into its present relative position among the Cornish 

 rocks. We looked in vain for the traces of such a fault in Gerrans 

 Bay. In a valley about half a mile north of Great Cam (fig. 2) we 

 at first supposed that we had discovered the line of fault ; but this 

 opinion was not based on any adequate evidence ; and the settlement 

 of this point must be left to future observers. 



3. Lastly, among beds so highly inclined and contorted as those in 

 the cliffs south of St. Austell, it is neither impossible nor improbable , 

 that the southern parts of the section may be presented to us in an 

 inverted position. Extensive cases of inverted sections are now 

 familiar to all geologists, and at present I am inclined to adopt this 

 hypothesis (not, however, without much hesitation) as perhaps more 

 probable than the former. 



The above-noticed irregularity of position between the slaty masses 

 and the granites is also seen on the east side of St. Austell Bay. From 

 the head of the bay, towards Lostwithiel, the beds skirting the granite 

 are mineralized and penetrated by innumerable veins ; and (on the 

 evidence obtained during a former year) I believe that one or two 

 very great faults range up the country towards Lostwithiel, If we quit 

 the mineralized district, and descend towards Gribben Head, we find 

 a prevailing dip towards the north : but at a point of the coast about 

 a mile south-west of Fowey, there is an anticlinal axis striking across 

 the headland ; so that the dip of the beds at Gribben Head is nearly 

 south-east, and at a high angle. Numerous fossils have been found 

 on the coast between Fowey and Gribben Head. From one single 

 quarry we collected about a dozen species, all of which were Devonian 

 without any admixture of Silurian or Cambrian types. Among the 

 Devonian species of this part of the coast may be mentioned the Port- 

 lochia latifrons, Bronn, sp. 



* Pal. Foss. Cambr. Mus. part 2, fasc. 1, p. 84. 



