BRITISH POSTGLACIAL 6- RECENT DEPOSITS. 385 



tween the two sets of deposits ; no appearance of a gradual 

 change from arctic to boreal and temperate conditions. The 

 beds which seem to afford some such evidence belong entirely 

 to the postglacial series. I thought at one time that we had 

 some trace of a passage from arctic to temperate conditions in 

 the brick-clays and Carse-deposits of the Forth, but the Carse- 

 clays have since been found in several places to rest unconform- 

 ably upon the arctic shell-beds. 



Dr. Gwyn Jeffreys has indeed described the occurrence at 

 Fort- William of old beach-deposits in which occur a number of 

 species of molluscs, such as Lacuna divaricata, Pleurotoma tur- 

 ricula, etc., that have a somewhat northern range, commingled 

 with the common forms of our present coasts, and one species, 

 Pecten islandicus, which is no longer a native of our seas, but is 

 widely distributed in the Arctic Ocean. But there is reason to 

 believe that there are really two deposits at Fort- William — the 

 one being of late glacial, and the other of postglacial age. I 

 am inclined, therefore, to believe that the unconformity between 

 the glacial and postglacial deposits, upon which Messrs. Cross- 

 key and Eobertson have insisted, 1 will be found to hold true for 

 Scotland generally. 



Among the most interesting and important of the Scottish 

 postglacial beds are the so-called " submarine forests and peat." 

 These vegetable-layers have been observed at many different 

 places at and below high-water mark. They vary in thickness 

 from a few inches up to four or five feet, and are made up prin- 

 cipally of the remains of trees and other land-plants. Sometimes 

 they repose directly upon true glacial deposits, in other places 

 they are underlaid by river-sand and gravel, and alluvial silt 

 and clay. In the estuaries of the Forth and Tay they clearly 

 belong to an older date than the raised-beaches and great Carse- 

 lands, since they everywhere pass underneath the marine and 

 estuarine postglacial beds. As there is no district perhaps where 

 they can be studied to better advantage than in the lower reaches 



1 "Monograph of the Post - Tertiary Entomostraca. " — Pcdceontographical 

 Society, 3874. 



2c 



