138 Correspondence — Rev. Canon Bonney. 



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QUAETZ DYKES NEAR FOXDALE. 

 Sir, — Though I have not visited Foxdale in the Isle of Man, 

 I venture to express a doubt whether Mr. Lomas (p. 34) has succeeded 

 in proving its quartz dykes to be igneous rocks. ' Dykes ' and veins 

 of that mineral are common in many countries, and cut almost all 

 kinds of rock, though, as might be expected, they are rather rare in 

 the more igneous or basic limestones. Sometimes the dykes attain 

 a considerable thickness and may be traced for a long distance ; at 

 others veins run off into the finest threads, and their demeanour is 

 unlike that of an igneous rock, from which they are often far away, 

 and their abundant fluid cavities and consequent whiteness (as 

 described by Mr, Lomas) suggest that they have been formed 

 from water. That silica, both crystalline and colloid, is so deposited, 

 especially from hot springs, is well known (see, for instance, a very 

 important paper by the late Mr. J. A. Phillips, published in the 

 Quarterly Journal of the Geological Society, vol. xxxv, p. 390). 

 The fact that at Foxdale quartz veins traverse the granite in itself 

 suggests they are later in origin and formed by thermal waters, with, 

 which their occasional relation to dykes of microgranite is quite 

 consistent. Mr. Lomas appears to regard the fact that the veins on 

 entering the granite change locally into pegmatite as strongly in 

 favour of his hypothesis. But the presence of felspar or even 

 mica in a vein does not prove it to have had an igneous origin. 

 I have examined numbers of quartz-felspar-mica veins in gneissoid 

 rocks which seemed to differ in important respects from granite. 

 Sometimes, though by no means always, they have been affected by 

 the pressures which have produced the schistose structure ; but the 

 three minerals are usually associated in a clotted and irregular fashion, 

 very different from that characteristic of rocks which have solidified 

 from a molten condition, and their structure frequently is abnormally 

 coarse, even in comparatively thin veins, where a true igneous rock 

 would be almost invariably either compact or not more than micro- 

 granular. The most remarkable case of mineral grouping which 

 I have seen was in the neighbourhood of Svolvaer in the Lofoten 

 Islands. Here a coarse gneissoid rock was cut by a quartz vein, 

 varying irregularly in breadth from two or three feet to as many 

 yards. By its side, and in places mixed with it, were a fairly broad 

 band of felspar and a much narrower one of a dark ferromagnesian 

 mineral, which at the time, more than thirty years ago, I took for 

 a pyroxene. The quartz was white and curiously divided by sharp 

 joints into parallelepipeds, rather variable in size. If, then, this 

 was an igneous vein, there must have been three distinct ejections 

 (only locally mixing) of quartz, felspar, and a ferromagnesian 

 mineral (not necessarily in the order of enumeration). The pegmatite 

 of the Foxdale vein, according to Mr. Lomas, contains felspars, 

 some over three inches long, perfectly formed, and showing crystal 

 faces. But the latter habit (except when there is considerable 

 difference between the fusion point of a mineral and the residual 



