174 INVERTEBRATE PALAEONTOLOGY 



The early Mesozoic history of the British area is 

 directly continuous with that of Permian times. During 

 an interval that may well have extended through half 

 the era, the terrestrial, mainly desert, conditions that 

 succeeded the Pennine uplift were maintained. En- 

 closed patches of water that survived through the 

 Permian period passed through the several stages of 

 desiccation until the whole area was covered with 

 shifting sand-dunes and wind-swept loess. The Rhaetic 

 stage had prevailed over Southern Europe long before 

 any modification of Triassic conditions affected this area. 

 Just before the close of the Rhaetic period, marine 

 encroachment and steady, though local, isostatic depres- 

 sion, brought the coastal waters of the Mid-european 

 sea over parts of " Britain," and let in a gulf from the 

 " Atlantic " in the north-western parts of the district. 

 The main Rhaetic coastline passed from the eastern 

 slopes of Dartmoor northwards to the neighbourhood 

 of Cardiff, and then took a north-easterly course, past 

 the south of the Pennines to North Yorkshire, with a 

 westerly extension towards the Cheshire plain. A large 

 island or peninsula covered much of Eastern and South- 

 Eastern England, so that the sea area was of the nature 

 of a strait widening at its two extremities. In no part 

 of the British area was it possible to get far from land. 

 The inroad of the sea over the Triassic desert-surface 

 resulted in adulteration of the water by solution of in- 

 crustations. The " Rhaetic Bone- Bed " and a curiously 

 limited Invertebrate fauna bear testimony to the 

 unhealthy quality of the Rhaetic coastal belt. 



The chief features of Rhaetic topography were 

 maintained through the Jurassic period all British 

 deposits of that stage having been formed within detrital 

 limits. Liassic strata are largely argillaceous; shales 

 and clays become interbedded with sands and littoral 



