38 
Correspondence — Mr. T. Mellard Reade. 
COEEESPOUDEITCE. 
THE THREEFOLD DIVISION OF THE BOULDER-CLAY OF THE 
NORTH-WEST OF ENGLAND. 
Sie, — Your correspondents Mr. Mackintosh and Mr. Morton raise a 
mucli 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 : 
Ist. By the distinctive character of the inclosed fossils. 
2nd. 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- 
ditious 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 bis 
communicatiou, it is true, does not. commit himself directly to any 
theory on the subject, but bis 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 
upper and lower clays, of cold more or less inten se, the middle 
gravels of a mild climate, or what are called interglacial conditions. 
This, I believe, is the theory generally accepted by those who upliold 
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. 1 
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 difiereuces existing between the deposits according 
to Mr. Morton, so far as I can gather from bis 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 
1 There are others who consider the whole to be the product of land-ice, while 
some deuy altogether the glaeial character of the beds, and consider them to be post- 
glacial clays reconstructed out of the pre-existing glaeial deposits. 
