PALEONTOLOGY. 151 



we are forced to believe that the climate of the British 

 islands must then have had a more tropical character. 

 Again, when in more recent deposits we discover re- 

 mains of the reindeer, musk-ox, and mammoth, and in 

 other beds of the same age certain species of mollusca 

 which now only inhabit the seas of more northern 

 regions, we have good evidence for concluding that 

 the climate of Britain was then very much colder than 

 it is at present. 



From a consideration of the character and abundance 

 of the fossil remains we may sometimes gain an idea of 

 the :ime that elapsed during their accumulation. The 

 formation of a bed of limestone crowded with organisms 

 of various kinds, many of the corals and crinoids being 

 still in their upright position of growth and surrounded 

 by the broken debris of others, together with the shells 

 of numerous species of mollusca, the formation of such 

 a bed must have occupied a long series of years. 



A bed of sand or sandstone will, on the other hand, 

 enclose a very different class of relics, if it contain any at 

 all, bivalve shells, mostly with separated valves, the 

 waterworn fragments of some corals and gasteropods, and 

 possibly a few drifted bones of some terrestrial animal, 

 may be found ; such remains indicate the rapid accumu- 

 lation of the materials composing the stratum, which in- 

 deed, although of the same thickness, may only have 

 occupied in its formation as many weeks as the bed of 

 limeston e did years. 



Other facts are also elicited by such a comparison ; 

 for many of the fossils occurring in the limestone belong 

 to genera which exist only in deep water, and the very 

 mode in which they occur shows that the water above 



