2 00 The Scottish Natiwalist. 



occasionally stones and larger erratics. The clays themselves 

 are often extremely tenacious, and have been worked for brick- 

 making. The stones vary in size, from mere small pebbles up to 

 fragments 6 and 8 inches in diameter. I have seen some, however, 

 which measured more than i foot across ; and one large one was 

 even 4 feet in thickness. The clays have all the character of the 

 well-known glacial clays, with arctic shells, but hitherto they have 

 not yielded any molluscous remains. I have already explained 

 the absence of organic remains in these clays,^ and shown that 

 this is sufficiently accounted for by the fact that at the time those 

 clays were being laid down, the upper reaches of the estuary 

 must have been nearly fresh, and greatly chilled by the influx 

 of swollen glacial rivers. The beach deposits of the same age, 

 which occur in less confined areas, and upon the sea-coast, con- 

 tain an assemblage of species similar to those now living in our 

 seas ; but the presence of the large Greenland whale tells of a 

 somewhat colder temperature, while the presence of ice-floated 

 stones and occasional disturbed bedding in the carse-clays, are 

 further testimony in the same direction. From these facts, and 

 certain other considerations which 1 cannot enter upon at pres- 

 ent, it may be concluded that the principal geographical and 

 climatic changes which supervened upon the close of the genial 

 post-glacial period were as follows : ist, A gradual submergence 

 took place, which brought the sea over some extent of country in 

 the south of Sweden, and again called into existence the German 

 Ocean and the English Channel, at the same time that the British 

 Islands became separated from each other, and were reduced to 

 a somewhat smaller extent than they now show. 2d, These geo- 

 graphical changes were accompanied by a gradual deterioration 

 of the climate, the Gulf Stream becoming considerably reduced 

 in volume, and the seas surrounding our islands acquiring in con- 

 sequence a lower temperature. To this date may be attributed 

 the demolition of many of the trees whose remains occur in the 

 peat of our maritime and upland regions, and doubtless some of 

 the buried forests in the deeper peat-bogs of the inland low-lying 

 districts ought to be assigned to the same stage. Peat-mosses now 

 began to increase, and to cover wide areas ; and perhaps 'the great 

 pine-trees in the peat of southern England flourished at this time. 

 The rainfall was probably considerably in excess of the present, 



^ Tt is only in the upper reaches of the ancient estuary where organic 

 remains appear to be wanting ; in the lower part of the Carse of Gowrie 

 Scrobiciilaria piperata occurs locally here and there. 



