92 Correspondence — Mr. 8. V. Wood, Jim. 



trouble us with such multitudes of these low things ? We poor 

 amateurs seem to be so many Pharoahs— to be afflicted in all our 

 quarters with protozoan creatures enough for all four of Egypt's 

 animated plagues ! — K. ex. 



MR. CROLL'S HYPOTHESIS OF THE FORMATION OF THE YORK- 

 SHIRE BOULDER-CLAY. 



Sir, — I submit that Mr. CroU's hypothesis of the formation of the 

 Yorkshire Boulder-clay by a sheet of land-ice, without any inter- 

 vention of the sea, is at once negatived by the abundant beds of 

 sand and gravel intercalated in it, in one of which occur shells per- 

 fect, unrolled, and sometimes double; and that his explanation of 

 the absence of chalk from the purple clay by one arm of his ice- 

 sheet having passed north of, and so escaped, the Wold is equally 

 negatived by the fact of this clay overlying and passing gradually 

 down (through clay with more and more chalk) into the chalky clay 

 along the Holderness coast, viz., at Dimlington cliff, and at the 

 cliff south of Mappleton ; for besides this gradual transition, these 

 places could not by any possibility be reached from the direction of 

 Shap without the chalk being crossed. 



From the way in which Mr. Croll uses the quotation from a paper 

 of mine as to the origin of the chalk in Boulder-clay, the reader 

 might suppose that, like Mr. Croll, I regarded such Boulder-clay as 

 a deposit of land-ice without intervention of the sea ; and I am 

 anxious not to be misunderstood in this respect : for though I regard 

 the material making up the chalky clay, and indeed most of that 

 making up all glacial clay, as the product of land glaciation, yet the 

 evidence seems to me unanswerable that such clay, wholly unstrati- 

 fied as it is, has been deposited under the sea; and that moreover to 

 all appearance as tranquilly as many sedimentary deposits. 



Were your pages less engrossed with Glacial topics, I should like 

 to discuss with Mr. Croll the evidence bearing upon this subject, as 

 well as upon the hypothesis of an ice-sheet 2,000 feet and more in 

 thickness, which he and Mr. Jamieson insist has passed, regardless 

 of hill and vale, over the higher mountains of Scotland, such as 

 Schiehallion, and the filling up of the North Sea with ice ; but until 

 opportunity offers, I must content myself with demurring to all these 

 propositions. 



I may mention that Mr. Eome thinks that the Shap blocks are 

 confined to the upper part of the purple clay, and that he ascertained 

 the exact spot (about 15 feet from the top of the cliff) near Saltburn 

 whence one of these boulders had come. Supposing this to be con- 

 firmed, it would show that the glacial period had nearly terminated 

 when these erratics came over ; and the period arrived when the 

 glacier ice having been lifted out of the straths and valleys, the 

 mountain regions had become an archipelago filled with ice-floes, to 

 the agency of which (and not to that of bergs) both Prof. Harkness 

 and myself refer the transport of the blocks in question — the period, 

 in fact, to which I would refer all the glacial accumulations of the 

 Scottish highlands. Searles V. Wood, Jun. 



