Correspondence — Mr. D. Mackintosh. 189 



tion in volcanic products would necessarily result from either source 

 of supply, and that if the central-fluid theory is to be maintained, 

 it must be for other and less fanciful reasons than those hitherto 

 adduced. I quite agree with the remark that it has been rather 

 assumed that geologists believe in a fluid interior with a solid crust. 

 They probably believe nothing of the kind, for, like other inquirers 

 accustomed to scientific investigations, they are not disposed to adopt 

 a belief, unless it be grounded on well-ascertained facts ; and as 

 there now exists but little faith in mere opinions of great authorities, 

 it would appear that in the present phase of the question, the most 

 satisfactory state of mind would be one of pure scepticism. 



S. Allpobt. 



CORRELATIGIif OF THE SCOTCH A^D ENGLISH DRIFTS. 



Sir, — As I have probably devoted more time to the examination 

 of the drifts of the N.W. of England than any other observer, a 

 few remarks from me seem to be called for by Mr. James Geikie's 

 article in your last number. While very willing to acknowledge the 

 great value of his contributions to Post-tertiary geology, I cannot 

 agree with him in regarding all the drifts of the above area as sub- 

 ordinate varieties of one great formation ; for the more these deposits 

 are investigated, the more one becomes convinced of the classificatory 

 value of tlae well-defined and more or less persistent sub-divisions 

 they present. 



The blue clay of the W. Eiding of Yorkshire, Cumberland, and 

 N. Wales, is not only distinct in .colour, and in the character of most 

 of its included stones, from the other clays, but it must have been 

 subjected to great denudation, leaving a deeply-undulating or hum- 

 mocky surface, before the lower brown clay was deposited. The 

 latter (though it embraces a considerable variety) difl'ers in its general 

 colour, composition, and relative proportion of local and erratic 

 stones, the number of large boulders, etc., from the upper clay ; and 

 its surface generally undulates as if it had been extensively denuded ; 

 while the surface of the upper clay is in most places a dead flat. 

 The gravel-and-sand formation between the two clays is not a series 

 of intercalations, but as persistent a deposit as either of the clays. 

 It has to a great extent been derived from local rocks, and the 

 number of erratic stones it contains is much smaller than in the clay 

 above or below. Its surface must have been deeply denuded before 

 the upper clay was deposited, as Professor Hull and Mr. De Eance 

 have shown. In the neighbourhood of hills it generally rises up 

 from beneath the upper clay, and forms a series of knolls or ridges. 

 There are few sand-and-gravel knolls that are not capped with de- 

 cided Upper Boulder-clay,^ or show indications of having once been 

 more or less covered with this clay. I do not believe that in the 

 area under consideration there are any sand or gravel knolls over- 

 lying decided Upper Boulder-clay, though they may occasionally 

 come above a subordinate clay bed of the middle drift, or a thin bed 



1 In the neighbourliood of Oswestry there are striking instances of high and 

 abrupt gravel-and-sand knolls capped with upper boulder-clay. 



