12 



KNOWLEDGE 



r January 1, 1897. 



the summit have been postponed until the autumn of 1896, 

 and will not be possible unless more efficient means are 

 found for transporting the equipment and for determining 



the time. 



__ ^ 



Urttrrs, 



[The Editors do not hold themselves responsible for the opinions or 

 Btat^ments of correspondents.] 



» 



To the Editors of Knowledge. 



SiKS, — In the light of Prof. Bonney's new publication, 

 " Ice Work, Past and Present," it is, I presume, per- 

 missible to advance any reasonable theory on many of the 

 moot points connected with the glaciation of England in 

 geological times. The darkness on these points is most 

 profoimd, and existing theories, according to the Professor's 

 judicial summing up, can occasionally be played off each 

 against another. 



With no desire to advance a theory, I should be glad to 

 obtain a discussion as to the conditions existing towards 

 the end of that period in which the great chalky boulder 

 clay of East Anglia was laid down. As an observer of 

 this deposit in Essex, perhaps I may be permitted to 

 submit a few notes as a result of practical work. 



First, as to the composition of the deposit : — It is to 

 be noticed that many of the softer rocks (chalk and harder 

 limestone) have undergone great abrasion, pointing to 

 glacier action. On the other hand, there is a large 

 proportion of the erratics in which the formation has 

 proved eminently preservative. I have in my possession 

 echini and other fossils which, although completely freed 

 whilst in the boulder clay from their chalky matrix, have 

 nevertheless preserved their delicate markings, and which 

 have apparently suffered as little in that disengaging 

 process as by the most refined artificial method. I have 

 also found dendritic markings preserved ou pieces of chalk, 

 as also striations on these very soft rocks. It will pre- 

 sumably be said that all ttiis goes to show that there has 

 been but little percolation of rainwater in subsequent 

 time, but it demonstrates a great deal more. It proves 

 that the conditions of transit of these fragments was at a 

 certain point rendered what we should call safe ; that is to 

 say, the bulk of the deposit was sealed up at a certain 

 stage and afterwards quietly deposited. 



Again we have to take into account the enormous mass 

 of this travelled ch'hris. Reduced now at places to a 

 fragment, it was once continuous over the greater part of 

 Essex, as also over Sufiblk and Norfolk. Attaining now 

 sometimes a thickness of near forty feet, it may be safely 

 inferred to have been considerably thicker in earlier times. 

 The conveyance of this enormous mass implies adequate 

 engines, and such (to refer again to the Professor's book) 

 ■would appear to be the floes generated at an " ice foot." 

 See pages C3 and 64 of " Ice Work.'' (That the deposit 

 was made during a submergence seems now to be granted 

 by all the best authorities.) From what point did the 

 majority of these ice-floes start '? Mr. Searles Y. Wood, 

 jun., as one reads his paper, would seem to think that the 

 greater proportion came from near what is now the 

 Lincolnshire coast. The point incident to our mquiry 

 is that they came from a northern direction. This does 

 not appear to be doubted. Again, the current that set 

 southerly must have been long engaged in this transport, 

 and on these shallow shores floe must have been dashed 

 upon floe in continuous rhythm ; none, so far as we know, 

 succeeded in returning north with their load. 



What was the configuration of this ancient sea ? Land- 

 locked at the south, as it would appear from Prof. 

 Prestwich's paper on the " Westletton Beds," and also by 



reference to j\Ir. Whittaker's " Geology of the London 

 Basin," where he speaks of high land to the south. 

 That is to say, both authorities would hardly allow the 

 Dover Straits to have had an existence at that time. Now 

 we come to another point where the light is not so clear — or 

 we might rather add, where the darkness is profotmd — and 

 that is the causes that brought about the close of this 

 deposition of travelled rock. Amelioration of climate due 

 to secular or temporary causes has been advanced in 

 explanation, and ultimately, of course, that agency was 

 triumphant. But, such a process being very gradual, we 

 ought, where an upper surface has been preserved, to see 

 something of a shading off of the boulder clay rather than 

 an abrupt termination. (It may be advanced that no such 

 upper surface has been preserved, but there is a post- 

 glacial gravel that cannot be separated from the same 

 submergence, which would therefore appear to preserve the 

 upper surface, i In all such cases the termination, ac- 

 cording to Mr. Searles Wood, is abrupt — and that is my 

 experience. 



I cannot find that any particular stress has been laid 

 upon what must have been once a powerful factor in 

 inducing local change in the North Sea, and that is the 

 opening of the Dover Straits. A moment's reflection will 

 show that at whatever period of the glacial age this breach 

 was made, an increase of temperatttre must have resulted 

 all along the seaboard. Assuming the south-western 

 current of warm water to have held along the same course, 

 so far as the South of England is concerned, as it does at 

 present — and we have no reason to suppose it did not — we 

 should have had in the extreme south-east two totally 

 different seas divided from each other only by the Wealden 

 anticline — one ice-laden and one comparatively warm. 

 Now let us suppose a breach to be made through the 

 chalk. This would be rather of a sudden uattire, but the 

 whole set of currents in the North Sea would be changed, 

 and all the shores of that cold sea would be laved once a 

 day by the initial tidal wave of warmer water. The 

 destruction among the icebergs would be great, and what 

 were not melted would be driven far north by the current 

 from the south. It does not seem possible to exaggerate 

 the importance of this change. The certain result would 

 seem to be a sudden termination of the glacial conditions 

 so far as the shores of the narrow sea extended. The 

 important question is, when was the breach made ? Much 

 earlier than the deposition of the chalky boulder clay it 

 could not have been (set' Prestwich and ^^"hittaker, already 

 referred to). Even without consulting these authorities it 

 does not seem credible that heavily laden ice-floes could 

 have existed had there been a breach at the south. Later 

 than the deposition of the chalky boulder clay it could 

 hardly have been, because a general elevation of the land 

 must have soon succeeded, as we know of only a thin spread 

 of marine gravel afterwards. It seems to be quite consis- 

 tent that this gravel, due to other currents than those 

 which brought the ice-floes, should accumulate for the 

 time the sea bed remained submerged, and would lie upon 

 the glacial ih'brts in precisely the manner we find it. 

 I should be glad to know how far such a problem as this 

 has been inquired into. J. French. 



Felstead, Essex. 



♦ 



THE SEXUAL SENSE AND THE X RAYS. 

 To the Editors of Knowledge. 



Sirs, — It occurred to me some nine months ago that in 

 the then recently discovered X rays we had an explanation 

 of the sexual sense of moths. Such a " sense " has long 

 been suspected, but the medium through which it acted has 

 hitherto been obscure, because it was fotmd to penetrate 



