282 DR. A. ttLLLIGAN 0~N THE PETROGRAPHY OF [vol. lxXV, 



whole thickness undoubted evidence of shallow -water conditions. 

 One need only adduce as evidence of this the types of fossils which 

 make up so large a part of the rock and the general broken 

 condition in which they are found. The great thickness which 

 the limestone attains in some parts is accounted for by its accumu- 

 lation upon a subsiding sea-floor. Whence was the calcareous 

 material derived? The old continental land which had been 

 subjected to denudation during Old Red Sandstone times must 

 have been considerably lowered by the supply of material for the 

 formation of those beds, further it seems most probable that the 

 rivers draining the old land would have base-levelled themselves so 

 far, and would be draining land of such low relief, that they would 

 no longer be able to bear along much material in suspension, and that 

 they would have assumed the characters of old rivers in which the 

 proportion of material in solution would be greatly increased. No 

 doubt much of the calcium carbonate was derived from pre-existing 

 calcareous deposits, the carbon dioxide merely acting as a carrier 

 of the calcium carbonate from land to ocean. To this cause, that 

 is, the solution and carrying away of the calcareous material, may 

 be attributed the abundance of chert-pebbles found especially in the 

 lower beds of the Millstone Grit, these representing the insoluble 

 parts of the limestone. 



But no area of pre -Carboniferous limestone would be of sufficient 

 extent to supply the calcium carbonate in this way : much of it 

 must, therefore, have been derived by a process of leaching, and so 

 it seems that a great amount of carbon dioxide drawn from the 

 atmospheric -ocean supply must be locked up in the Carboniferous 

 Limestone. The bearing of this will be dealt with later, when the 

 question of the climate of Millstone Grit times is discussed. Here 

 it is sufficient to note that this process of leaching would result in 

 a thick cover of rotted rock being left over the low ground and 

 along the river-courses of the old continent. There is little 

 evidence for determining the directions of the rivers of the Carbon- 

 iferous Limestone period, but they must have been chiefly from the 

 northern continent, the quantity of material accumulated rendering 

 a large drainage-area necessary. The Yoredale Beds which succeed 

 the Carboniferous Limestone in the North of England represent 

 the removal of the rotted material from the old land-surface by 

 reason of the rejuvenation of the rivers caused by uplift at the 

 source, and its being carried forward to be spread out and deposited 

 upon the old continental shelf upon which the lime-secreting 

 animals of the Carboniferous Limestone period had flourished. 

 This removal of the covering laj^ers of the soil would prepare the 

 land for yielding a fresh supply of unleached rock, and it is only 

 by the upheaval of such a prepared land-surface that it seems 

 possible to satisfy the conditions demanded by the Millstone Grit. 

 That uplift of the great Northern Continent was taking place, 

 producing instability and establishing a line of weakness along its 

 southern edge, the direction and position of which is almost exactly 

 denned by the northern edge of the Central Valley of Scotland, 



