YORKSHIRE NATURALISTS’ UNION AT GRASSINGTON. 267 
fault, was made very slowly, and took an immense period of time. 
The scene of it was at one time the bottom of a sea. Corals were 
then hard at work, each little animal doing his little share in the 
immense work of building up the crust of the earth. The encrinite 
rooted himself among the corals, and reared his head slowly, waving 
his arms through the water in search of food. The numerous kinds 
of brachiopods attached themselves by means of their self-made 
strings to different objects. Fishes swam in the waters, and water- 
snails and trilobites crept about, acting as scavengers. The shells 
and hard skeletons of all these animals went to build up the reef, 
rapidly, for he would die if he got too far beneath the surface of the 
water, and similarly with the other animals. But with all this, as age 
after age went on, the depression gradually began to go onat a greater 
rate than the reproductive power of the animals whose shells went to 
build up the thick mass of what is now limestone, and instead of 
forming a long reef of equal height, the work became broken up into 
rows of detached portions of reef, and as the bottom went on 
lowering the water became too deep to sustain the life of even these 
animals, which had outlived their fellows, and struggled on to build 
up these islets. Not only had the water been too deep, but fine 
black mud was being washed very slowly into the deeper part, killing 
outright the animals which, of all others, required clear water for 
their existence, and this mud brought with it fronds and parts of ferns 
and other plants, so that even these islets began to be interred in 
mud. ‘The soft open parts of the corals crumbled away, and the 
upper hard layers of shells began to bend over and sometimes to 
break off and slide down into the muddy bottom ; but in any case 
it went to form a dome-like cap over the main body of the islet. 
The deposit of mud became more rapid, and in time interred the 
entire reef. All this time the beds on the other or upper side of the 
fault, although they had been lowering, had done so so slowly that 
animals whose dead shells went to make up the strata had a hard 
struggle for existence, and at length died out, partly because the 
water was too shallow, and partly because the animals were choked 
with mud. These latter deposits, extending from this fault into 
Northumberland, are known as the Yoredale Rocks, and are well 
eveloped in the valley of the ae 
Sept. 189t. 
