SHELL BEDS. 45 



ment of glaciers. This boulder-clay, or "till," has long been 

 recognized as existing in Arran ; it is seen in fine section in 

 the lower seaward portions of many of the gleus, especially 

 in the south and south-west of the island, and is probably 

 spread out, though in thinner layers, over much of the sur- 

 face between the river channels, which are of course best 

 adapted for the retention and exhibition of detrital matter. 

 The deposit consists of several beds, with distinct and per- 

 sistent characters, which are often overlooked, so that the 

 whole is regarded as one formation. The lowest portion 

 next the natural rock is an unsti-atified clay, of a red or 

 chocolate brown colour, in which large boulders, both angular 

 and rounded, and usually striated, are thrown together pell- 

 mell. Over this are beds of laminated clay and unstratified 

 sandy clay, with very few boulders, but occasionally striated 

 pieces of rock. This is the chief repository of the arctic 

 shells; but over it are other beds of a character somewhat 

 different, among which occur shell beds with species such as 

 now inhabit our own shores. 



Mr. Smith, of Jordanhill, was the first (Mem. Wern. Soc., 

 Jan. 26, 1839) to distinguish between the shelly deposits of 

 the West of Scotland to assign them to two distinct ages 

 to recognize the arctic character of the lower, and thence to 

 infer that a sub-arctic climate once prevailed in this country, 

 connecting with that of the shells other evidences of glacial 

 action. This was an important generalization; it at once 

 introduced order into the heterogeneous mass of beds, ranged 

 conflicting facts and appearances harmoniously under one 

 head, and formed a new era in post-tertiary geology, like 

 that established among the tertiary beds by the generaliza- 

 tions of Lyell. 



The credit of the first discovery of boreal shells in Arran 

 is due to the Eev. Eobert Boog Watson, B. A., F.B.S.E., of 

 Edinburgh, who has kindly furnished the annexed list of the 

 shells. A detailed account of the beds and their fossil con- 

 tents was laid before the Royal Society of Edinburgh, and 



