﻿38 Correspondence — Mr. T. Mellard Reade. 



OOIiK-ESIPOlsriDSIETOE. 



THE THREEFOLD DIVISION OF THE BOULDER-CLAT OF THE 

 NORTH-WEST OF ENGLAND. 



Sir, — Your correspondents Mr. Mackintosh and Mr. Morton raise a 

 much wider question than the one immediately contained in their 

 letters, and it is impossible for me to really discuss the nature of 

 the North Dock Sections without inquiring into the foundations of 

 the theory upon which their classification of these deposits rests. 

 Having in a former letter expressed an opinion that there is not 

 sufficient evidence to justify in this case the threefold division of the 

 Boulder-clay, will you permit me to state more fully my views on 

 the subject. 



First then it will be necessary to inquire by what characters a 

 geological subdivision is usually recognized, — there are three : 



1st. By the distinctive character of the inclosed fossils. 



2 ad. By persistent lithological character and continuity over a 

 considerable area. 



3rd. By constant relation to well-defined and known deposits 

 above and below. 



It is evident that these involve the prevalence of physical con- 

 ditions differing when each deposit was laid down. These conditions 

 may have differed widely and extended over great areas, or may 

 have been of a more restricted and local nature. Mr. Morton in his 

 communication, it is true, does not commit himself directly to any 

 theory on the subject, but his position involves it all the same. 



Mr. Mackintosh, on the other hand, boldly states that the Lower 

 Boulder-clay, Middle Sands and Gravels, and Upper Boulder-clay, 

 are each representatives of considerable variations of climate, the 

 tipper and lower clays, of cold more or less intense, the middle 

 gravels of a mild climate, or what are called interglacial conditions. 

 This, I believe, is the theory generally accepted by those who uphold 

 the threefold division of the Boulder-clay, though they differ as to 

 the nature of the lower clay, some attributing to it a subaerial 

 origin, and others considering it to be an older marine deposit.' 

 Unfortunately the terms are often used so loosely, that it is not 

 always possible to interpret what is really meant by them, though 

 if the division is to be upheld, they must mean something. 



Having examined the general principles, and attempted to extract 

 the signification of the terms. Lower Boulder-clay, Middle Drift, 

 and Upper Boulder-clay, let us see what support is lent to the theory 

 by the Boulder-clay Sections at the Liverpool North Docks. 



The distinctive differences existing between the deposits according 

 to Mr. Morton, so far as I can gather from his letter, are that the 

 Upper Clay contains fewer stones than the Lower, and is worked with 

 the spade : while the Lower Clay is more closely packed with small 

 stones, and has in consequence to be worked with the pick. The 



^ There are others who consider the whole to be the product of land-ice, while 

 some deny altogether the glacial character of the beds, and consider them to be post- 

 glacial clays reconstructed out of the pre-existing glacial deposits. 



