330 



THE GEOLOGIST. 



is a bed of wliich. the limestone is liard and dolomitic, and also finely 

 laminated, there being more than a dozen laminae to the inch ; this 

 is its normal character, but where it is penetrated by a pipe it looses 

 its hardness, and becomes dull and earthy ; the laminae also, which 

 in the unaltered portion of the pipe are firmly coherent and difficult 

 to separate, are here easily split open, the surfaces of their planes 

 being highly decomposed ; the colour, too, of the decomposed portion 

 is somewhat changed, being of a light yellow, while the limestone of 

 the bed generally is of a brown or grey hue, and so soft has it become 

 where it forms the surface of a pipe that it crumbles to pieces on 

 being touched, and for half an inch in it can be cut with a pocket- 



Lign. 5.— Z*, Sand ; c, Decomposed surface of Limestone. 



knife. In fig. 5 the decomposed surface of a pipe is shown for the 

 sake of illustration. When two pipes are very close together, and 

 are only separated from each other by a thin wall, as sometimes 

 occurs, the limestone composing it is afiected throughout. The 

 upper surface of the limestone upon which the sand rests has suffered 

 in like manner ; and it sometimes happens that a loose piece of lime- 

 stone rests upon it, and its surface is also just as much decayed, and 

 the same is the case with the surfixce of the large boulders embedded 

 111 i saiul when they are of mountain limestone, but boulders of 

 irap not affected in the least. 



JNluch of the limestone in which the pipes are excavated is crys- 

 i alii lie, some of the beds being highly dolomitic. The thickest beds 

 in Nvhicli thoy occur have a ct)ncretionary structure, exhibiting those 

 peculiar coralloid forms for which the upper member of the mag- 

 nesian limestone is so famous. On the south side of the quarry, 



