^2 American Association, 



for this extinction of species aud the sudden appearance of a gi-eat 

 number of othersylie could not say. He bi-ought this subject before 

 the Association to heai^, if possible, some suggestions as to th& 

 illustration of this important question. In the Lower Silurian we 

 have 445 species and 150 genera; in the Upper Silurian 464 

 species and 150 genera^ while 14 genera pass from the Lower into- 

 the Uppevy and 43 species. There is here a considerable link, yet 

 the break is remarkable, and is marked by strong ])hysical uncon- 

 formity. In the Devonian we have 415 species and 131 genera. 

 There is apparently a much more ge-ntle passage from the Upper 

 Silurian to the Devonian than from the Lower to the Upper Silu- 

 rian, yet only 8 species and 60 genera pass into the Devonian. 

 In the Carboniferous we have 1646 species and 302 genera. The 

 passage from the Devonian to the Carboniferous is easy, yet only 

 58 species and 43 genera pass into this formation. It is a remark- 

 able circumstance that there is here a great decrease in the 

 development of life. In the Permian we have only 157 species 

 and 78 genera, while only 37 genera and 5 species pass from the 

 Carboniferous into the Permian. When we reach the bunter 

 sandstone formation we have no fossils at all in Britain, so that 

 there the break is complete. In the Keuper we have 18 species 

 and 15 genera. In the Lias 454 species and 129 genera ; of these 

 there pass into the Oolite 62 genera and 6 species. In the Lower 

 Oolite we have 994 species and 224 genera, 89 genera and 36 

 species of which pass into the Middle Oolite. The Middle Oolite 

 yields 107 genera and 264 species, and the Upper Oolite 130 

 genera and 218 species. From the Middle to the Upper Oolite 

 there pass 30 genera and 9 species, while 36 genera and 5 species 

 pass from the Oolite to the Cretaceous ; 49 genera and 16 species 

 pass from the Lower Cretaceous to the Upper Cretaceous. In 

 the Upper Cretaceous times we have a great development of life, 

 viz., 1275 species and 314 genera. In the Middle Eocene we 

 have 977 species and 274 genera. We have no Miocene in Bri- 

 tain. In the Pliocene we have 631 species and 202 genera ; and 

 from the Pliocene there pass 50 genera and 236 species to the 

 Pleistocene. With respect to physical breaks in the foregoing 

 formations, the Permian rocks lie quite unconformably on the 

 Carboniferous, and the New Red on the Permian ; and though in 

 any one locality where the marine Cretaceous beds lie direct on 

 the Oolite, they appear conformable; yet the local occurrence of 

 intervening breaks in the Purbeck and Wealden beds shew the 



