CHAP. IX.] JURASSIC PERIOD. 153 



It by no means follows, however, that this elevation was 

 uniform and equable over the whole area. We notice, in 

 fact, a decided tendency to the formation of separate basins 

 of deposition, and to the upheaval of certain submarine 

 ridges having a general east and west direction across 

 England. Thus we can hardly understand the continuance 

 of shallow water over the Mendip district without suppos- 

 ing a local upheaval, and it is not unlikely that there is an 

 eastward extension of this axis in the form of a ridge sepa- 

 rating the southern basin from that of Gloucestershire. 

 A second ridge stretched across North Oxfordshire and 

 Northampton, the evidence given on p. 138 proving that 

 over these counties there was a broad space of shallow 

 water separating the deeper basin of the Cotteswold area 

 from the equally deep north-eastern basin. 



This ridging up of the Liassic sea-floor, and the conse- 

 quent formation of three separate submarine basins like 

 jthose which exist in the modern Mediterranean, must have 

 produced very material changes in the physical conditions 

 of the Jurassic sea, and was probably one reason why the 

 waters of the southern basins became so rapidly clear 

 enough for the growth of reef-building corals ; the muddy 

 material would be all thrown down in the northern basins, 

 that of Yorkshire on the one hand and of the Irish sea on 

 the other, the submarine ridge preventing the currents from 

 carrying much of it over the central and southern counties. 



It is clear, however, that this was not the only cause for 

 the cessation of the shaly and muddy sediment, and it is 

 probable that the supply of such sediment was diminished 

 by a slight elevation of the country from which it was 

 obtained. Assuming that the supply was chiefly derived 

 from the erosion of the Coal-measure shales (as previously 

 suggested), it is certain that the amount of such material 

 carried off a given area by one system of rivers must have 

 become less and less as the rivers cut their way through 



