THE MATERIALS OF STRATA. 35 



iaria clay of science, 50 named from a bivalve 

 II found in it, which lives in the swampy inlets 

 on the east c<>a>t of England. In that clay are 

 occasionally found the remains of walrus and 

 and grampus; showing that the inlet 

 known as the Wash extended southward during 

 the deposition of the ciay, over tiic lower peat in 



much of tlie Isle of Ely. When peat becomes 



I by the deposition of superincumbent 

 ilidated like the rocks with which 

 it alternates. There are important geological de- 

 ls, which h An m the same way, in the 

 >f time. At the beginning of 

 the tertiary period, at Bovey rraceyin Devonshire, 

 alternations of lignite and day form a succession 

 lich uii up a lake-basin in the older 

 rocks : Mm seen in the Brack- 

 lisham ' the Isle ^i Wight. In the second- 



I remarkable bed of vegetable 



matter live feet thick, at Ilrora in Sutherland, 



which is worked for coal. Thinner beds are found 



on the Yorkshire coast, which appear to have 

 An like the modern beds of peat, in the posi- 

 tions in which they are found. Far more impor- 

 tant are the beds of consolidated vegetable matter 



found m the upper carboniferous rocksof the pri- 

 mary period, which :mionly known as coal. 

 They often give evidence of change in level of 

 land during their accumulation ; the same bed 

 being thick in one place and divided up at a 

 little distance by intervening sedimentary de- 

 posits. These accumulations of sediments pre- 

 serve indications of the plant life of the earth, 

 and in the associated - its are occasional- 

 ly found remains of insects and other terrestrial 

 animals which lived in the >ame epochs of time. 

 3 



