GEOLOGY 



of large horn-shaped corals of the Clisiophyllum type, and the other is 

 equally full of Prodticius giganteus, the largest of Brachiopod shells. 

 These layers are quarried, where the fossils are most crowded, for orna- 

 mental purposes, as the stone takes a good polish, and many arc the 

 churches and other public buildings throughout the kingdom in which 

 the Stanhope and Frosterley marbles, as they are called, display their 

 beautifully preserved organic remains from the old Upper Bernician or 

 Yoredale sea. 



(15) Elsie. 



(16) Rose-Mary, or The Pea Post. This layer is a mass of 

 Lithostrotion corals in their original position of growth. The sections of 

 the corallites are the ' peas.' 



(17) The Mucky Posts. 



(18) Crabby. A ' crabbed ' or difficult stone to work. 



(19) Toby Giles. And finally 



(20) The Fine Posts. 



The topmost portion of the Great Limestone is often irregularly 

 bedded, presenting the aspect of ellipsoidal blocks of stone with inter- 

 vening calcareous shale. This appearance may be due to what Mr. J. G. 

 Goodchild has called the ' dwindling ' of the limestone, or its gradual 

 decay under the effect of solvents. To this structure is no doubt owing 

 the name of ' Tumbler Beds,' often given to this part of the formation, 

 the word ' Tumbler ' meaning ' boulder ' in the local dialect. The ex- 

 traordinary persistence of the Great Limestone makes it without excep- 

 tion the best and most convenient datum-line in the Lower Carboniferous 

 deposits of the north of England. 



Sandstones and shales, together with a very thin and by no means 

 constant representative of what to the north and west is, under the name 

 of The Little Limestone Coal, perhaps the most continuous seam of coal in 

 Britain (as it certainly is the most constant of the Bernician seams, 

 stretching from the northernmost portions of Northumberland to the 

 Craven district), separate the Great from 



No. 145. The Little or Second Limestone. — This is the Little Lime- 

 stone proper referred to under No. 204. In it the lead veins have fre- 

 quently been found to yield very abundant ore, but it is a thin and, in 

 this county, not very regular bed. 



Sandstones and shales, the last of these non-calcareous intervals, 

 lead to 



No. 121. The Fell Top Limestone, a still thinner and more variable 

 limestone, sometimes duplicated by means of intercalated thin shales and 

 sandstones, and sometimes absent altogether (though in that case usually 

 represented by a calcareous shale full of ordinary limestone fossils, amongst 

 which trilobites are common). This is the highest marine limestone in 

 the Carboniferous Series of Durham ; and although the Geological Sur- 

 vey, owing to the necessity of carrying on lines of division decided on 

 further south, have been compelled to fix the upper boundary of the 

 Limestone Series a little above this horizon, there is no such necessity 

 I 92 



