576 Correspondence — 3fr. D. Mackintosh — Prof. T. G. Bonner/. 
The above theory of the tripartite origin of the Boulder-clays will 
explain many facts which, at first sight, are calculated to puzzle the 
observer, and make him think that there is no constant order of 
succession in the drift-deposits of the north-western plain. The 
current-distributed subglacial clay could not, everywhere, have been 
equal in amount. It is indeed reasonable to suppose that in various 
places, and at various times, there would be a partial or even entire 
failure in the supply of this clay, so as to leave the sea nothing to 
deposit but loam, sand, or coarse local grit, which would often con- 
tain few or no erratics, as the currents which would bring the clay 
would likewise be the principal carriers of the floating coast-ice 
(part of which, however, would appear to have been blown in 
aberrant directions by wind). 
In the lower clay, there is often a change in its character upwards, 
as if the supply of subglacial clay and erratic stones had been 
diminishing ; in otlier words, it often becomes less stony, and more 
intercalated with loam or sand towards its junction with the non- 
glacial middle sand. This indeed miglit be expected on the sup- 
position that the lower clay and middle sand were deposited during 
the same submergence. 
The tripartite theory likewise explains the degeneracy of the 
clays southward from the source of supply ; and upward on the 
hill-slopes where the clays gradually become more earthy and local 
in their character ; for the progress of the submergence must have 
melted the ice upward, so as to limit its extent and consequent 
power of supplying subglacial clay. D. Mackintosh. 
COLOURING OF OOLITIC ROCKS. 
Sie, — An excellent illustration of Professor Judd’s remark on the 
colouration of the Oolites, quoted at p. 480, from “ The Geology of 
England and Wales,” may be now seen in a cutting on the Midland 
Eailway to the north of Kettering. The bed is very low down in 
the Inferior Oolite, doubtless part of the Northampton Sands. It 
seems to be (I have only noticed it from the train, but I believe it 
identical with rock I have elsewhere examined) a soft sandstone, 
perhaps calcareous. Yertical joints divide its beds into blocks, so 
that there is a rough resemblance to courses of masonry. Sometimes 
these blocks are wholly brown ; but in other cases the heart of a 
block is blue-grey, while the exterior for several inclies is brown : 
so that it is evident that the former was the original colour, and that 
atmospheric water, as may be seen in so many other cases, has con- 
verted the pyrite (or, what is here more probable, the carbonate of 
iron, vide Judd, Geol. Butland, p. 136) into limonite. The effeet 
produced by this change along the planes of bedding and of jointing 
is very singulär, something like masonry exceedingly coarsely 
pointed. T. G. Bonney. 
FiKRATUM. — In the Rev. T. G. Bonney’s article, Geol. Mag. Nov. 1877, p. 499, 
lines 8 and 14, for “ Hungary,” read “ Bohemia.” 
