COLLYWESTON SLATE. 17 



hood, and when first dug up they are of a light grey colour, which on 

 exposure soon turns to a dark brown; they endure very well, and are more 

 ornamental than the ordinary Blue Slates. The splitting is caused, says 

 Professor Phillips, by organic exuviae, on which account, as you would 

 naturally suppose, immense numbers of fossils are found imbedded, princi- 

 pally consisting of Trigonnellites and Gervillidae, one of the latter (Gervillia 

 Monotis) being the characteristic fossil. I have also found there Myacitis 

 ninomformisy Ceromya concentrica, Hinnites tegulatus, Pinna cuneata, 

 Natica (Grandis, ?) and the rare Pteroceras Bentleyii. A great deal of 

 wood, sometimes in immense pieces, is found in the strata; and the layers 

 of Slate often present ripple marks, rain drops, and tracks of crawling 

 worms. 



The laminations are pretty regular throughout, and the following is a 

 section taken from the largest pits. 



ft. in. DESCRIPTION. 



Red Sandstone, . . Considerable depth. 



The Gigs, (so called by the workmen,) are discoidal masses of stone, 

 exceedingly hard, imbedded in the sandy strata; they vary very much in 

 size, some being not bigger than a cart wheel, while others are twenty 

 feet or more in diameter. What these masses are, and how formed, 

 perhaps some of your more scientific readers will explain; they are of a blue 

 colour, and so hard that it is very difficult to break them with a hammer. 



These Slate beds have never yet been found to the north of the Welland. 

 On crossing that river, which runs, as before stated, through the Lias, you 

 come again to the same outlier of the Lower Oolite, at its eastern extremity, 

 at Ketton, which there consists of broad deep beds of the great Oolite 

 rock, in much request for buildings, and presenting the following 

 section : — 



VOL. VII. d 



