io6 Physical Geography. 



The presence of land plants in the very uppermost 

 Silurian strata, as, for instance, near Ludlow and May 

 Hill, indicates the neighbourhood of land. The 

 physical geography of the area was rapidly changing, 

 marking the beginning of an evident Continental 

 epoch. The subject is of so much importance, and 

 when first propounded was considered to be such a 

 dangerous innovation on established views, that I 

 shall give the reasons in some detail, making use for 

 that purpose of passages from my memoir ' On the Red 

 Eocks of England, of older date than the Trias,' pub- 

 lished in the ' Quarterly Journal of the Geological 

 Society,' in 1871. 



The circumstances which marked the passage of the 

 uppermost Silurian rocks into Old Red Sandstone seem 

 to me to have been : First, a shallowing of the Silurian 

 sea by accumulation of sediment, aided by slow up- 

 heaval, which gradually produced a great change in 

 the physical geography of the district, so that the old 

 marine area became changed into a series of mingled 

 fresh and brackish lagoons, which finally, by continued 

 terrestrial changes, were converted into lakes ; and the 

 occurrence of a very few genera or even species of fish 

 and Crustacea, common both to the fresh and brackish 

 or even salt waters, does not prove that the Old Red 

 Sandstone is truly marine. At the present day, animals 

 that are commonly supposed to be essentially marine, 

 are occasionally found inhabiting fresh water. In the 

 inland fresh lakes of Newfoundland, seals are common. 

 They breed there freely, and never visit the sea. The 

 same is the case in Lake Baikal in Central Asia, and 

 it is on record that the inhabitants of the shores of the 

 Sea of Aral, now brackish, were in old times clad 

 in sealskins got from the seals that inhabited those 



