CHAPTEK IX. 



THE BH^ETIC PERIOD. 



PERHAPS no part of the series of English strata gives clearer 

 proofs of the progress of critical knowledge, or encourages stronger 

 expectations of additional discoveries by continued observation in 

 new localities, than the horizon which we have now reached. 

 Formerly the line of division between the new red marls and the 

 liassic deposits was drawn with entire confidence : a mere glance 

 at the unproductive red marls and sandstones was enough to send 

 many geologists away from them to the more congenial blue clays 

 and limestones full of fossils, which had the aspect of a new creation 

 of life. The first geologist to express an opinion that the so-called 

 liassic beds contained in their lowest part fossils of an earlier type 

 that they were, in fact, triassic as far as the fishes and reptiles 

 were concerned, was Sir Philip Egerton. His observations applied 

 specially to the rich bone-bed of Aust, which yields, among other 

 well-determined fishes, Acrodus, Ceratodus, Gyrolepis, Hybodus, 

 Nemacanthus, and Saurichthys, the species being on the whole of 

 a triassic rather than liassic type ; and, among reptiles, Plesiosaurus 

 and Ichthyosaurus. 



Important as these observations were, they would hardly have 

 occasioned the introduction of a new term for a new group of strata 

 intermediate between the new red and the lias ; but they were 

 strengthened by discoveries in the Tyrolese and Swiss Alps, where 

 deposits were found at Hallstadt, St. Cassian, and Kossen, which 

 yielded abundance of fossils, and among them ammonites of pe- 

 culiar character, the whole appearing to represent a fauna inter- 

 mediate between the trias and the lias, as previously known. Thus 

 evidence was presented on a large scale of a long period, previously 

 only conjectured, with a great series of ascertained inhabitants of 



