CHAP. IX.] JURASSIC PERIOD. 135 



because they mark the epoch when the great Triassic lake 

 was first invaded by the sea. At, or near the base of the 

 Ehaetic shales, there is usually a layer of shaly sandstone, 

 which contains phosphatic nodules and is crowded with the 

 remains of fish and small reptiles, and sometimes there are 

 several such layers. It would appear as if the sudden 

 irruption of the sea- water was prejudicial to the inhabi- 

 tants of the Triassic lake, so that most of them died, and 

 their bones, scales, and teeth were drifted into layers on 

 the sea-floor. 



The Ehsetic Beds have been found everywhere in Eng- 

 land where the junction of the Trias and Lias is exposed, 

 and they occur in the north-east of Ireland, but are not 

 known in Scotland. It is true that on the western Scot- 

 tish coasts the base of the Lias is nowhere clearly exposed, 

 but Professor Judd thinks that no representative of the 

 Rhaetic shales exists there. These beds, therefore, so far 

 as our present knowledge enables us to judge, were con- 

 fined to the area of the Anglo-Hibernian lake. On the 

 borders of this area, as on the Mendip Hills and in Gla- 

 morganshire, the shales and limestones are sometimes re- 

 placed by sands and sandstones. 



The Lias has a wider extension ; its thickness is often 

 more than 1,000 feet, and it must have overlapped the 

 Trias more or less in every direction, though the actual 

 extent of the Lower Lias may not have been very much 

 greater than that of the Trias, because the western coast of 

 the Triassic lake was in many places very steep. The 

 broad outcrop of the Lias stretches across England from 

 Dorset to Yorkshire, and outlying tracts occur in Stafford- 

 shire, Shropshire, and Cumberland, and in the north-east 

 of Ireland. They occur also on both sides of the Scottish 

 Highlands. 



The thick clays and shales of this series indicate a sea 

 into which many rivers discharged a constant supply of 



