152 NEOZOIC TIME. [CHAP. IX. 



for explanation, and it may be fairly assumed that this 

 material was mainly supplied by the destruction of the 

 Coal-measure shales. Large tracts of Coal-measures must 

 have existed at this time both in Ireland and Scotland, and 

 the rivers flowing off these tracts would pour little else 

 than black mud into the surrounding sea, while the waves 

 would eat deep into such portions as came within their 

 reach during the gradual submergence. 



Middle Jurassic Time. The change from Liassic shales 

 to Oolitic limestones and marls is a rapid one, and is ac- 

 companied by a great change in the fauna, for though 

 many Liassic species range into the passage beds (Midford 

 Sands), yet very few survived till the era of the Inferior 

 Oolite. These facts imply that a considerable and rapid 

 change took place in the physical conditions of the period, 

 but what this change was is not quite so easy to determine. 

 At first sight nothing seems easier to explain than such a 

 change ; a clay succeeded by a limestone seems to point to 

 a general depression, whereby the extent of the sea was 

 enlarged, and its depth increased ; but it is an error to 

 suppose that all limestones are deep-water formations, 1 and 

 oolitic limestones in particular are generally of shallow-water 

 origin. Moreover, the Stonesfield Beds, with their proofs of 

 shallow water and the vicinity of land, occur in the middle 

 of this Oolitic series, the whole of which becomes more and 

 more estuarine in its character as it is followed to the north- 

 east. These facts prove conclusively that the water was not 

 on the whole so deep as it had been in Liassic times. Lastly, 

 in Oxfordshire the Upper Lias and Inferior Oolite seem to 

 thin out beneath the Great Oolite, and there are also signs 

 of erosion between the Inferior Oolite and the Lias ; these 

 facts imply an elevation of the sea-floor and not subsidence, 

 for in the latter case the Inferior Oolite would overlap the 

 Lias. 



1 See " Physical Geology," Part L, ch. xii. 



