FOSSIL CORALS. 85 



owing to the lovely and variable tints of the lime- 

 stone, which range from grey to yellow, pink, and 

 red. Thin slices mounted on glass for the microscope 

 form exquisite objects. The student may obtain for 

 himself any quantity of coralliferous rock in and 

 about Torquay. By moistening the dried and perhaps 

 worn surface of the rock, the included corals become 

 plainly visible. Devonian corals are also obtained at 

 Plymouth, Teignmouth, and Ilfracombe. 



Speaking of British fossil corals, perhaps it would 

 be impossible to direct the student to richer fossili- 

 ferous deposits than the lower Carboniferous strata of 

 Scotland. Mr. James Thomson, F.G.S., is of opinion 

 that their abundance in Scotland is due to the strata 

 of the latter having been deposited in shallow water, 

 whilst the English Carboniferous or " Mountain " lime- 

 stone was laid down in deep water. But the great 

 thickness of the limestone in Derbyshire (about four 

 thousand feet) indicates a depression of the sea- 

 floor all the time the beds were forming ; for its 

 mineral characters would have been altered if it had 

 simply filled up an ocean basin to that depth. As we 

 have already seen, a gradually lowered sea-bed has 

 been stated by Darwin to be necessary to continuous 

 coral growth. 



Of all the Carboniferous corals the genus Zaph- 

 rentis is one of the most widely distributed and 

 generally abundant. It is usually found in a very 

 perfect condition, and may often be seen in the walls 



