November 23, 1893] 



.VA TURE 



77 



stand under what conditions it can become an excavating one, 

 ard how it can hollow out basins, &c. 



When ice moves away from the slope which gives impetus to 

 a glacier, the motion rapidly slackens and presently stops. The 

 distance travelled over the level ground is a function of the 

 weight of the glacier, of ihe amount of the slope, the friction of 

 its bed, <S:c... i.e. of the elements making up the visa tergo ; but 

 in the very largest glaciers, so far as observation goes, the 

 motion rapidly ceases on level ground. This is the evidence 

 wherever the phenomenon has been observed and reported 

 upon. 



This being so, I altogether question not only the arguments 

 of those who champion the excavation of lake basins by ice, 

 but also of that laiger school who invoke movemenis of ice 

 over level plains of many hundreds of miles in extent in order 

 lo explain the diift phenomena. They do it, so far as I know, 

 on the ground that ihey cannot appeal to any other cause with- 

 out doing injustice to that modern metaphysical bogey, " The 

 Doctrine of Uniformity." My small boy might just as well, 

 on the same principle, attribute the excavation of his )-Orrirger 

 to the porridge in the bowl. True rock basins were no doubt 

 very largely due to the weathering of rocks v\hich exfoliate, and 

 whose structure is not homogeneous. This is a very old ex- 

 planation, but like many sober old inductive truths it is not so 

 attractive nowadajs as an appeal to the imagination, com- 

 bined with a' good, sturdy, consistent loyalty to some tt prion 

 ]>ostulate, which would have won the hearts of the old school- 

 men. Henry H, Howorth. 



30 Collingham Place, Cromwell Road, November i6. 



Rock Basins in the Himalayas. 



There is one statement in the interesting communication of 

 my colleague, Mr. T. D. LaTouche, which seems to require 

 qualification. After a tolerably extensive experience of the 

 Himalayas, I should be inclined to say that rock basins are of 

 lairly frequent occurrence, of all sizes from the largest to the 

 smallest, but they are almost without t xception filled with stream 

 deposits, and only occasionally -can their formation have been 

 due to glaciers ; for they are usually found where there are no 

 traces of glacial action to be seen, and at levels to which we 

 have no reason to suppose that glaciers ever reached. In the 

 liills of eastern Baluchistan, where the rainfall is much less than 

 in the Himalayas, rock basins more or less filled by recent sur- 

 face deposits are even more common, and here their origin by 

 deformation of the surface can generally be established. The 

 same cause probably accounts for the Himalayan rock basins, as 

 there are abundant proofs that the elevatory movement has been 

 far from uniform, and that the variations in its intensity have 

 been both extensive and often extremely local. There are fre- 

 quent occurrences of suiface deposits which appear to have 

 originally been formed in rock basins, but have since been cut 

 into by the streams, owing to the corrasion of the barrier, and 

 we may attribute the absence of lakes in the Himalayas to the 

 rapid current and large burden carried by the streams, in con- 

 sequence of which they have been able to fill up the basin, and 

 often to corrade the barrier, as fast as it was formed. 



R. D. Oldham. 



" Composite " Dykes. 



Prof. Jui>d's excellent paper in the current issue of the 

 Quarterly /otcrnal of the Geological Society (p. 536) calls to mv 

 mind some common and similar examples among the " elvans " 

 of Cornwall (whicli are dykes in the ordinary acceptation of the 

 term), and but little has tieen published offering some explana- 

 tion of their bea ing on urroundinij rocks. I have observed, 

 notably in the district of Cligga Head (nine uiiles N. W. of 

 Truro), the marked difference between the structures exhibited 

 by dykes in the parts in contact with the rock through which 

 tfiey intruae (in the Cligga instance Devonian slate), and their 

 centre, amounting almost to a rock distinciion. 



In the appended sketches I have endeavoured to illustrate 

 my meaning from ac ual instances. 



Fig. I represents a section of an elvan or dyke outcropping 

 slightly to the north of Cligga promontory, and from its position 

 apparently connected with the main mass of Cligga Head granite, 

 k bursts through the slate. The centre {l>) of the dyke con- 

 sists of a rock of homogene'>us tex'ure, quartzo-felspathic base, 

 and some scattered p arphyritic felspar crystals. The sides (a a) 



NO. I 256, VOL. 49] 



in contact with the slate {s i) show a rock of apparently similar 

 base, but shot with long a^icular crystals of schorl, the whole 

 rock being of a very dark colour, due probably to the presence 

 of wolfram. 



Fig. 2 is a section of a very common form of Cornish elvan, 

 consisting of alternate laminre of granite {(id) and " schorl rock," 

 that is, rock consisting of schorl and quartz, generally in about 

 equal proportions {cc). 



These bands are very common in the slates and in the granitic 

 bosses. Further, an analysis of a typical " schorl rock " of this 

 class showed a silica percentage of Gy6 {vide Judd's l aper, 



s ^ 



Fic. I. 



p. 1545), and of a typical granitic band of 74 "8 (De La Beche, 

 " Report on Geology of Cornwall, Devon, and West Somer- 

 set," p. 189). It is very doubtful, however, if either of the above 

 instances is a case of a d}ke putting on such differences in 

 mineralogical and chemical character in its several parts as to 

 amount to a difference of rock species. 



As De La Beche points out, the schorl rock may be simply a 

 f raniie in which the felspar and mica are replaced by schorl. 

 An instance, however, of a rock one may call " a dyke within a 

 dyke " is the Cligga mass itself, which is nothing but a gigantic 

 d)ke. De La Beche, in his work above cited (p. 164), has 

 figured it. The d}ke is so strikingly split into layers as to 



appear stratified, the hard comparatively small-grained layers 

 standing out in bold relief from the contiguous layers of more 

 easily decomposed rock with their large porphyritic felspar 

 crystals. 



Besides the difference in size of the felspar crystals, the harder 

 rock is much darker in colour (being of a red hue) than the 

 softer, which is pale pink and in places whitish. These physical 

 differences, however, count for little in drawing a distinc- 

 tion of rock species between the layers, and I was unfor- 

 tunately unable to avail myself of any published analyses of the 

 different parts, but their superficial characters are so distinct as 



