456 STRATIGEAPHICAL GEOLOGY 



occupied so large a part of Northern Europe in later Triassic time. 

 The level of the water in the Inland Sea seems to have been some- 

 what below that of the outer ocean, because the Rhsetic Beds here 

 and there pass beyond the limits of the Keupei 1 , but the difference 

 of level cannot have been great, and the invasion of the area by 

 the outer sea seems to have been accomplished quietly and with 

 little disturbance of previously formed deposits. The sea-waters 

 simply occupied the basins and embayments of the great lake, in 

 which shales and limestones began to be deposited. 



It is clear, however, that the mingling of sea and salt -lake 

 waters resulted at first in the death of many of the creatures which 

 inhabited both. The fish and marine reptiles which came in from 

 the sea could not exist in the highly saline mixture, and their 

 remains are found in the bone-bed which so often occurs at or near 

 the base of the Rhsetic Beds. Neither could all the Mollusca which 

 came in with them flourish in the land-locked waters, and those 

 that managed to exist are small in size, as if dwarfed by unfavour- 

 able conditions. 



In some localities there are several bone -beds at successive 

 horizons, suggesting that there were several invasions of the sea 

 over the barrier which separated the inner from the outer waters. 

 Such irruptions may have taken place at conjunctions of high tides 

 and southerly winds. Moreover, in Alsace and Lorraine, and again 

 in Glamorgan, the Rhsetic sandstones and bone-beds are overlain 

 by a band of red marls exactly like those of the Keuper, from 

 which we may infer that communication with the outer sea was 

 not yet permanent, and that there was a time when the area of 

 the inland sea was again diminished by evaporation so that portions 

 of it returned to the condition of separate salt lakes. 



At length a further subsidence brought in such a body of sea- 

 water that permanent marine conditions were established, and 

 many forms of life were able to inhabit the northern water-spaces. 

 It was under these conditions that the shales and limestones of the 

 Lower Lias began to be deposited. 



The material of the shales was probably obtained from the 

 erosion of Palaeozoic shales, and more especially from those of the 

 Coal-measures, large tracts of which must then have existed as part 

 of the continental land to the west and north of the Liassic Sea. 

 During the Permian and Triassic periods the climatic conditions 

 had been like those of Central Asia at the present day, with a 

 small rainfall, but in Liassic time it is evident that the climate 

 was greatly changed, and that much more rain fell over the whole 

 region, so that active erosion went on, and the rivers carried much 

 mud into the Liassic Sea. 



