242 SECONDARY LIMESTONES. 



which may hereafter furnish us with more accurate 

 grounds of reasoning respecting yet unsuspected 

 changes in the earth's surface, during the deposition 

 of the whole series from the magnesian limestone up- 

 wards ; while, as far as the present facts go, I shall 

 immediately attempt an enquiry into this neglected or 

 forgotten subject. A future more accurate know- 

 ledge of the Lias, better reasoned on than it has y e t 

 been, may also ultimately throw light on the very 

 singular circumstances attending the red marl. At 

 present, I must suffer this hint to be pursued by future 

 observers ; but -the organic remains which have led 

 to it, are chiefly the vegetable ones and the Lacertae, 

 of which two have been termed Ichthyosaurus and 

 Plesiosaurus. Turtles and fish also occur in the Lias, 

 together with some crabs ; and among other shells, the 

 genera ammonites, nautilus, belemnites, scaphites, 

 helicina, trochus, tornatella, melania, modiola, unio, 

 cardita, terebratula, gryphsea, ostrea, pecten, echinus, 

 with encrinites and corals, and vegetable remains 

 including ferns and other terrestrial plants. 



The Oolithe, here distinguished from the lias, is a 

 term used to include the principal calcareous deposits 

 of Europe ; but, for the reasons so often assigned, we 

 have no right to identify them with the limestones of 

 other parts of the world. The great calcareous ridge 

 of the Jura is ranked here : but I need not enumerate 

 localities to be found in every geological work, whe- 

 ther as to this or the lias. English geologists have 

 extended this term to the whole deposit, calcareous or 

 otherwise, between the lias arid the ferruginous sand; 

 and thus, in England, it presents numerous calcareous 

 strata, to many of which peculiar local names are ap- 

 plied, separated by beds of the several other substances 

 which contribute to form the mass of the secondary 



