644 W. M. Sutchings — On some Lake-District Rocks. 



mass, and some apparently orthoclase porphyritic crystals, occur also 

 near Keswick. 



It is not at all unlikely that undoubted trachytes may be found in 

 the Lake District. Sir A. Geikie, in his address above referred to, 

 alludes to the general resemblance of these lavas to the " porphy- 

 rites" of the Old Eed Sandstone in Scotland, and Dr. Hatch quotes 

 the authority of Professor Judd for the fact that "excellent sanidine- 

 trachytes " exist among these rocks at Haddington ("Introduction 

 to the Study of Petrology," p. 100). Some lapilli seen in certain 

 tuffs also suggest that good trachytes exist somewhere among the 

 Lake rocks.^ 



Sir A. Geikie states that from the observations of himself and 

 Dr. Hatch on the rocks of the Lake District, he concludes that 

 lavas are much more abundant among them, relatively to "ashes," 

 than was allowed by Mr. Ward in his Survey Memoir and other 

 writings. In this conclusion it is probable that most workers in 

 the district would now agree. But there are, of course, many cases 

 in which it does not seem possible to decide whether they are flows 

 or ashes, either in the field or with the microscope, — certainly not 

 with the latter. The difficulties of this kind of examination are 

 very fully (not to say a little gloomily !) put forth in a paper by 

 Mr. Eutley (" On Community of Structure in Eocks of Dissimilar 

 Origin," Q.J.G.S. vol. xxxv. 1879), and anybody who may be in 

 danger of underrating these difficulties, or of getting too much 

 confidence in his power to solve them, or indeed to solve any sort 

 of questions whatever in microscopic petrology, would do well to 

 read that paper. 



The chemical and mineralogical changes which have taken place 

 in these rocks are of very great interest in some cases. They are, 

 however, most fully seen in the detrital rocks, — the ashes and 

 tuffs, — and I hope to return to some of them in connexion with 

 a study of the ash-slates, etc., of the district. 



1 Prof. Cole points out (" Aids in Practical Geology ") that the older trachytes 

 (or quartzless porphyries, orthophyres, etc.) may often be difficult to mark off from 

 the rhyolites, and suggests that the marked absence of porphyrite quartz in some of 

 the "altered rhyolites" may give rise to a suspicion that they were trachytes. 

 There is a " rhyolite" exposed in the plantation in front of Sbap Wells Hotel, which 

 differs notably from the other rhyolites near to it. Macroscopically it shows crystals 

 of felspar more prominently than any of the others do. Microscopically it shows 

 an abundance of felspar-laths and microlites, all orthoclase, lying in a rather plenti- 

 ful cryptocrystalline base, "speckly " in polarized light, apparently devitrified glass. 

 Such laths and microlites are absent from the other local rhyolites. There is none 

 of the spherulitic structure so abundantly developed in these rhyolites, and there is 

 no sign of free quartz or of " granophyre." The silica -percentage of a specimen 

 collected by me is 61-15, as contrasted with over 75 per cent, found in rhyolites of 

 Wasdale Head and Stockdale by Mr. Garwood. The microscope shows nothing 

 whatever to lead to the supposition that the silica-percentage has been lowered by 

 decomposition or infiltration, the rock being very free from chlorite, etc. This was 

 apparently never a rhyolite, but was most probably a trachyte with a large amount 

 of glassy base. 



