360 G. W. Bidman—On Underclays. 



thus forming separate layers, cause the latter to split up when 

 consolidated. 



(2) By the periodical supply of sediment — as by floods occurring 

 at intervals — so that one layer gets settled down and partially 

 solidified before the arrival of the next supply. 



Both causes of lamination are absent in the deep sea zone of 

 deposition ; the particles of mica are too small, and the supply of 

 sediment too continuous. 



And the nature of the beds immediately associated with under- 

 clays is at times suggestive of a deepish water origin. Thus we 

 frequently find a sandstone succeeded in ascending order by a shale, 

 next to which comes an underclay, upon which rests a bed of coal, 

 as in the following examples from the Yorkshire Coal-field : 



(1) (Coal 



Haigh Moor Coal \ Dirt 



(Coal 



Underclay and ironstone nodules 

 Soft black shale 

 Shale and ironstone 

 Sandy shale and sandstone 

 (Survey Memoii's, p. 714.) 



(2) Coal 



Underclay 



Shale and ironstone nodules ... 



Sandstone 

 (Survey Memoirs, p. 712.) 



And a glance through the sections given in the above Memoir shows 

 that such an order of succession is exceedingly common — it may, 

 indeed, almost be said to be tj'pical — while those above the coal 

 show not infrequently an inverse order, as in a portion of the Seaton 

 Carew boring given below : 



Grey sandstone .. . 

 Dark-grey sandy shale ... 



Coal ... ... 



Dark-brown fire-clay ... 

 Dark-brown sandy shale 

 Dark-grey sandy shale ... 

 White sandstone 



The obvious and usual interpretation of such a succession beneath 

 the underclay would be a change from the shallow -water sandstone 

 conditions, through the deeper zone of shale to still deeper whei-e 

 the fine-grained underclay was laid down. Above the coal, again, 

 the natural interpretation would be shallowing water. And the 

 following section from the Lower Carboniferous of East Lothian is 

 instructive : 



Limestone 10 feet. 



Seam of coal, 10 inches, and fire-clay under. 



Shale. 



White limestone. 

 (Survey Memoirs, East Lothian, p. 54.) 



The lower limestone contains marine fossils : Prodnctiis sp., Aihjris 

 ambiyua, Bhynchonella plturodon, Lithoatrotion juncexim, and L. irregu- 



