94 MR. e. e, walkep, on the gaknet-beakinu [Feb. 1904, 



AVe are face to face with the old difficulty pi distinguishing 

 between a rhyolite and a rhyolite-tuff, for these ' streaky ' rocks 

 show many points of resemblance to those acid rocks. Ward 

 mapped all these rocks as ashes, but at the same time he pointed 

 out that so thick a mass of rock would be sure to contain small 

 lava-flows which it would be almost impossible to trace in the 

 field. 



If the garnets of these rocks are examined, it is found that they 

 are not always perfect in form, but are often surrounded by 

 a white ring which, under the microscope, is seen to consist 

 of plagioclase-crystals growing out from the garnet-margin in 

 good crystal-forms. The garnet is always very much corroded ; 

 the projecting portions are often curved, and lose their isotropic 

 character at the extremities. Fragments of garnet occur in the 

 felspars (#819), showing the same polarization-tints as the felspar, 

 and only differing in refractive index. 



A compact green rock from Eagle Crag in the Langs trath valley 

 (3824) shows the association of felspar and garnet exceedingly well. 

 Very little garnet remains, but the fragments are surrounded by eight 

 or ten idiomorphic plagioclase-crystals of the same species as that 

 which occurs in the rest of the rock. Iron-ore, probably ilmenite, 

 has separated out in rods and irregular masses, and has been largely 

 converted into a brownish sphene. Flakes of chlorite also occur. 

 The streaks are formed by lines of felsite-material : these, as they 

 approach a large felspar in the neighbourhood of the garnet, seem 

 to become discontinuous and exhibit folding and crumpling. This 

 suggests that the felspar was formed after the streaks of feJsitic 

 material. The formation of ferromagnesiau mineral, as a result of 

 the corrosion of garnet, is rarely seen in the ' streaky ' rocks, but 

 is fairly common in the garnet-intrusives. 



Garnets, with perfectly-developed faces, occur in an ash at the 

 base of the 'streaky' rock on Rosthwaite Fell. In section (3836) 

 the garnet is seen to be breaking up at the margin, felspar being 

 produced. The original outline of the garnet can, however, be traced. 



A study of the garnet-intrusives has led us to believe that the 

 association of felspar and garnet affords evidence for 

 the originality of the garnet, the felspar having been pro- 

 duced by the action of the still liquid matter en the garnet. Garnets 

 surrounded by felspar-growths also occur in undoubted ashes and 

 breccias, above the series of ' streaky' rocks (3834 & 38-lf5). It 

 would seem, then, that this association affords no criterion for 

 discrimination between a lava and an ash; and the fact that it occurs 

 in true ashes throws doubt on the interpretation put upon it in the 

 case of the intrusive rocks. In the examples of ' streaky ' rooks 

 mentioned above certain features seem to point to the formation of 

 the felspar after the consolidation and compacting of the rock. 



It might be urged that the garnets with a felspar-ring which 

 occur in the ashes are fragmental. A glance at the felspar-crystals 

 is sufficient to convince one that this is not the case : for they often 

 show very perfect crystal-outlines, with sharp and deep re-entrant 



