2 POST-TERTIARY ENTOMOSTRACA. 



This task is not without its difficulty, since many clifFerent writers upon the Ghicial 

 Epoch have employed the same terms, without meaning the same things. Three terms 

 we shall at once discard. The term Northern Drift is far too loose to afford any help in 

 the identification of the position of a bed ; Avhile one of the peculiar characteristics of 

 the clay of the Clyde district, in which the fauna of the epoch is most perfectly preserved, 

 is that it is in no sense whatever a drift, but one of the most gradually and quietly 

 formed of deposits. The term Bride Clay is equally objectionable, since clay capable 

 of making bricks may belong to any period, and the brick clay of one part of the 

 country may be geologically distinct from that of another. The use of the term Till is 

 also in danger of misleading through its employment by the older Scotch geologists in a 

 very vague sense ; in fact, whenever a bed of coarse silt, or clay, or sandy loam was 

 found containing boulders it was called Till. 



While retaining the term " Boulder Clay," we cannot employ it without considerable 

 explanation. If we were to content ourselves with noting that certain species of 

 Ostracoda occur in the "Boulder Clay," or beneath the "Boulder Clay," without speci- 

 fying the precise character of the deposit to which we refer, it would lead to great 

 confusion. Under the general term "Boulder Clay " a very miscellaneous collection of 

 deposits, attributable to different causes and belonging to various periods of the great 

 Glacial Epoch, has been vaguely included. Any clay containing boulders is not 

 necessarily identical with the oldest Boulder Clay of Scotland. A fossiliferous Arctic 

 clay may contain boulders (as boulders may be found scattered through the Paisley Shell- 

 beds), and yet cannot be described as Boulder Clay without an extreme misuse of terms. 

 Boulders have been in all probability dropped into the Shell Clay from ice floating over 

 the Eirth of Clyde, precisely as they are dropped from the ice floating at the present day 

 in the St. Lawrence, and this Shell Clay, into which the boulders fell, may have been 

 resting upon an old " Boulder Clay '* and have been formed long subsequently to its 

 deposition and by entirely diflFerent agencies. A "■ Boulder Clay " may have been 

 carried from the land into the sea by a glacier breaking off into an iceberg, and thus 

 have covered littoral Shell Clays, and have become mixed with fragments of shells, 

 although not itself of marine origin. 



Without theorizing, therefore, upon the origin of the various beds roughly included 

 under the general terms " Northern Drift," '' Brick Clay," " Till," " Boulder Clay," it is 

 necessary to note their characteristics in order that the physical geologist may understand 

 the exact positions in which the Ostracoda we catalogue occur. ^ 



1 These deposits have been treated of to some e.xtent in the series of Memoirs on tlie Post-tertiary 

 fossiliferous beds of Scotland, by Messrs. Crosskey and Robertson, in the 'Transactions of the Geological- 

 Society of Glasgow,' vol. ii, p. 267 ; vol. iii, pp. 113, 321 ; and vol. iv, pp. 40, 128. See also Mr. Cross- 

 key's paper on the Boulder Clay, ibid., vol. iii, p. 149. 



