124 TOWN GEOLOGY. [v. 



were laid down after all this, I say, some change took 

 place in the sea-bottom, and brought down on the 

 reefs of coral sheets of sand, which killed the corals 

 and buried them in grit. Does any reader wish for 

 proof of this ? Let him examine the " cherty," or 

 flinty, beds which so often appear where the bottom 

 of the millstone grit is passing into the top of the 

 mountain limestone the beds, to give an instance, 

 which are now quarried on the top of the Halkin 

 Mountain in Flintshire, for chert, which is sent to 

 Staffordshire to be ground down for the manufac- 

 ture of china. He will find layers in those beds, 

 of several feet in thickness, as hard as flint, but 

 as porous as sponge. On examining their cavities 

 he will find them to be simply hollow casts of in- 

 numerable joints of Crinoids, so exquisitely preserved, 

 even to their most delicate markings, that it is plain 

 they were never washed about upon a beach, but have 

 grown where, or nearly where, they lie. What then, 

 has happened to them ? They have been killed by 

 the sand. The soft parts of the animals have decayed, 

 letting the 140,000 joints (more or less) belonging to 

 each animal fall into a heap, and be imbedded in the 

 growing sand-rock; and then, it may be long years 

 after, water filtering through the porous sand has 

 removed the lime of which the joints were made, and 

 left their perfect casts behind. 



So much for the millstone grits. How long the 

 deposition of sand went on, how long after it that 

 second dep osition of sands took place, which goes by 

 the name of the "gannister," or lower coal-measures, 

 we cannot tell. But it is clear, at least, that parts of 

 that ancient sea were filling up and becoming dry 



