330 
THE GEOLOGIST. 
is a bed of which the limestone is hard and dolomltic, and also finely 
hxminated, there being more than a dozen lamina) 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 laminiB also, which 
in the unaltered portion of the pipe are firmly coherent and difficult 
to separate, are lierc easily split open, the surfaces of their planes 
being highly decomposed ; the coloui', 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 liue, and so soft has it become 
where it forms the surface of a pipe that it crumbles to pieces on 
bemg touched, and for half an inch in it can be cut with a pocket- 
Ligii. 5. — b, Simd ; c. Decomposed sm-racc of Limestone. 
knife. In fig. 5 the decomposed sui-face 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 affected throughout. The 
upper sui'face of the limestone upon which the sand rests has suffei'ed 
in like manner ; and it sometimes hajipens 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 surface of the large boulders embedded 
in the sand when they are of mountain hmcstone, but boulders of 
trap are not affected in the least. 
Much of the limestone in which the pijjes are excavated is crys- 
talline, some of the beds being highly dolomitic. The thickest beds 
in which they occur have a concretionary structure, exhibiting those 
peculiar coralloid forms for which the upper member of the mag- 
nesian limestone is so fiunous. On the south side of the quarry. 
