470 Mr. Austen on the 



specimens ; the Favosites* ? ramosa, however, appears as plentifully as at Plymouth 

 and Chudleigh. 



11. Igneous Rocks. (Map, PI. XLI. Sect. PI. XLII. figs. 1. to 4, and 6.) 



Earliest Epoch. 



§ 1 . Interstratified. — The earliest proofs which we perceive in South Devon of 

 igneous agency are the numerous bands of hornblendic matter and ashes inter- 

 stratified wuth the older slate rocks. 



In the coast-section at Babbacombe, trap rests on beds of shale ; and near the 

 level of the sea (fig. 1.), the trap presents a rounded undulating surface, to the 

 outline of which other beds of shale conform ; but the enclosing strata, neither 

 above nor beneath, afford any indications of intrusion, or alteration by heat. 

 Limestone shale, succeeded by compact limestone, followed the deposition of the 

 trap ; the whole series dipping in one direction, and the disturbances which affect 

 this part of the country being clearly of more recent date than the trap. Mr. 

 De la Beche, it must be admitted, considers the trap at this place to be intrusive, 

 and to be the cause of the dislocations ; but in the numerous instances which South 

 Devon presents of undoubted eruptive masses, the mode of association is very 

 different. All we have here is a gradual change from one rock into another, which 

 cannot be better given than in the words of Mr. De la Beche's description : " The 

 trap becomes so altered in its character as it ascends, that the highest portions 

 scarcely deserve the name, presenting, where not in contact with the hmestone, 

 a base that effervesces, contains green specks and iron pyritesf." 



To the west of Babbacombe the beds of trap again rise, and are followed by a 

 much more considerable thickness of compact greenstone, which arching over 

 forms the hill known as Black Head ; and the sea-cliff affords a section of the 

 igneous rock with the underlying shales, &c. At Islam Farm this mass of trap, 

 much reduced in thickness, is interposed between slates below and limestone above ; 

 a similar position is to be observed at Chapel Hill, near Tor Moham, where in 

 several places the trap can be traced passing from a bright green hornblendic rock 

 into a harsh limestone with seams of ashes. From its very unequal thickness, this, 

 and other similar masses, were probably erupted very near the spots at which we 

 now find them. 



In the neighbourhood of the little hamlet of North Whilborough, 3^ miles west of 

 Babbacombe, igneous and aqueous rocks are similarly associated ; the trap putting on 

 various forms and acquiring considerable thickness. In some places it is a compact 



* Caunopora of Phillips, Pal. Foss. Cornwall, &c., 1841. 

 t Geol. Trans., 2nd Series, vol. iii, p. 168. 



