262 
HUGHES : IXGLEBOROUGH. 
are chiefly made up. Where corals grow there is not much rock 
exposed to denudation, and the waste of coral reefs and of the 
calcareous shells of creatures that live on it furnish the whole of 
the material which is heaped up on the sea margin and spread 
over the sea bottom, forming various textured Umestones instead 
of the shale and sandstone and conglomerate we meet with 
elsewhere. 
In a sinking area many changes would be brought about 
by variations in the form of the ground which was being depressed, 
and the sediment would be sorted accordingly. Thus we find 
places where encrinite stems have been swept together with 
hardly any other organism among them ; in another, great 
shells lie as they would best resist the current, with their edges 
Fig. 1. 
SECTION OF BASEMENT BEDS OF MOUNTAIN LIMESTONE ON BALA BEDS, 
BLACK ARK, NEAR THE HEAD OF INGLEBOROUGH TARN. 
buried and their round back only exposed, at other times the 
convex side is down and the white rim of the shell appears 
tlirough the black mud. The result of all this is, that, although 
there is a general uniformity through hundreds of feet of lime- 
stone, when we examine the sections in greater detail, and look 
for characters that may enable us to recognise and trace the beds 
from place to place, we find differences of texture and structure 
often persistent over very considerable areas. 
In the ascending section from Clapham, after crossing the 
fault at the head of the tarn, we start from the very base of the 
Mountain Limestone where it rests on the Bala Beds, near Black 
Ark. It is seen in a little stream which issues from a keld and 
