802 



THE WONDERS OF GEOLOGY. Lect. VIII. 



texture, which may be subdivided into the four following 

 formations, commencing with the lowermost : — 1. Various 

 crystalline slates, resting immediately on the granite of 

 Skiddaw Forest, and forming the base of the whole stra- 

 tified series. 2. Black glossy clay-slate, sometimes passing 

 into greywacke. 3. Green quartzose roofing slate, asso- 

 ciated, in every variety of complication, with felspathic 

 rocks of porphyritic structure. 4. Greywacke slate, often 

 more or less calcareous, and having subordinate beds pass- 

 ing into impure limestone, full of organic remains.* 



The absence of fossils in the older porphyritic slates, may 

 probably be attributable to the obliteration of all vestiges 

 of organic remains from the high temperature to which 

 they have been exposed; or animals may not have been 

 capable of living in an ocean exposed to continual incur- 

 sions of igneous matter. j* But Professor Sedgwick is of 

 opinion, that there is a line in the descending series of 

 strata in our Island, where organic remains entirely dis- 

 appear ; and that this line is by no means co-ordinate with 

 mineral changes induced by igneous action. 



27. Keview of the Silurian and Cumbrian Sys- 

 tems. — In conclusion, I will briefly review the leading 

 phenomena which have been brought under our notice in 

 the course of this Lecture. 



The strata comprised in the Silurian system, present all 

 the usual characters of marine sedimentary deposits. The 

 fossils comprise immense numbers of extinct crustaceans, 

 and of brachiopodous mollusca, some marine worms, 

 and many cephalopoda, crinoidea, and corals ; a few pla- 

 coid fishes, are the only vestiges of vertebrated animals ; 

 and fucoid plants the sole indications of the vegetable 



* Professor Sedgwick on the Structure of the Cumbrian Mountains, 

 Geol. Trans, vol. iv. p. 45-68. 



+ Professor Sedgwick on the Structure of large Mineral Ml 

 Geol. Trans, vol. iii. p. 469. 



