﻿576 Corresijondence — Mr. D. MacJcinfosh — Prof. T. O. Bonney. 



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 other words, it often becomes less stony, and more 

 intercalated with loam or sand towards its junction with the non- 

 glacial middle sand. This indeed might 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 tipicard 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. 

 Sir, — 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. Vertical 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 inches 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. Eutland, p. 136) into limonite. The effect 

 produced by this change along the planes of bedding and of jointing 

 is very singular, something like masonry exceedingly coarsely 

 pointed. T. G. Bonney. 



Erratum. — In the Rev. T. G. Bonney's article, Geol. Mag. Nov. 1877, p. 499, 

 lines 8 and 14, /o»- " Hungary," read " Bohemia." 



