MODIFICATION OF THEORY EXAMINED. 115 



(during the time of the formation of the 40-feet raised 

 beach and the deposition of the Carse-clays) the climate 

 was much colder than it is at present. The seas sur- 

 rounding our island appear to have had a lower 

 temperature than they have at present ; and our 

 Highland valleys seem to have been occupied by 

 local glaciers.* 



The Carse-clays of Scotland are best developed in 

 the valleys of the Tay, the Earn, and the Forth. 

 These deposits consist of finely laminated clays and 

 silt. "Now and again," says Professor J. Geikie, 

 " the deposits consist of tough tenacious brick-clay, 

 which does not differ in appearance from similar 

 brick-clays of glacial age." The clay is usually free 

 from stones, but occasionally blocks of six inches or 

 a foot in diameter are found in it ; and Professor 

 J. Geikie mentions having seen one four feet in 

 thickness. Stones of this size in a fine laminated 

 clay evidently indicate the presence of floating ice. 

 But, as Professor J. Geikie remarks, " it is rather the 

 general character of the clays themselves than the 

 presence of erratics which indicates colder climatic 

 conditions. The fine tenacious brick-clays are not 

 like the dark sludge and silt which now gather upon 

 the estuarine bed of the Tay, but resemble and in 

 some cases are identical in character with the lami- 

 nated clays of true glacial age with Arctic shells." 

 These Carse-clays, as he further remarks, appear in 

 a large measure to be made up of the fine " flour of 

 rock" derived from the grinding action of glaciers 



* In a paper " On the Obliquity of the Ecliptic," read before the 

 Geological Society of Glasgow in 1867, I concluded that at the time 

 of the deposition of the Carse-clays the mean winter temperature 

 was probably 10° or 15° lower than at present, and the Gulf Stream 

 considerably reduced. See also 'Climate and Time,' pp. 403-410. 



