82 MB. E. B. WALKER ON THE GABNET-BEABING [Feb. 1904, 



others besides, all of varying texture. To a large extent they 

 determine the remarkable gullies on Yewbarrow summit — namely, 

 Great Door and Little Door. 



An excellent example of these uralite -diabases is to be seen 

 about 80 yards uorth-north-west of Kirkfell Tarns (3907). Ophitic 

 structure is well shown, slightly-purple augite occurring in the 

 centre of the uralite-plates. The uralite-fibres tend to change into 

 biotite and serpentine at their margins, and minute biotite-tlakes 

 are also developed in the centres of the uralite-plates. This trans- 

 formation of uralite into biotite has been noticed above in the case of 

 the Criscliffe-Knotts dyke (p. SO). The fact that the change, how- 

 ever it ma}* be produced, occurs in a typical diabase lends support 

 to the conclusion that the intermediate mica-porphyri te 

 rocks of Criscliffe Knotts have been produced by the 

 intermixture of an acid granophyric rock with the 

 basic diabases. That some sort of intermingling takes place 

 is evident from the study of the Blea-Crag rocks, but there the 

 rocks have been much altered and do not often show the original 

 minerals. Alternative theories will be dealt with below. 



This Kirkfell dyke contaiued a xenolith of what appeared to be a 

 'streaky' rock. A section (3844) of this xenolith shows a banded 

 f e 1 s i t e, with abundant mica-Hakes grouped in lines. Quartz-grains 

 surrounded by a spherulitic growth of felspar are found, together 

 with sheaf-like aggregates of felspar-fibres resembling variolitic 

 structure. This felsite has its nearest analogue in the banded 

 felsite of .Burtness Combe. Buttcrmerc (see p. 84). 



It seems, then, as if all these basic dykes were given off from the 

 Eskdale Granite. Grey quartz -porphyries are very typical 

 marginal modifications of the Eskdale Granite. They are seen to be 

 intrusive into purple ashes and Eycott Lavas south of Tirockshaw Gill, 

 the intrusive rock sending veins into the metamorphosed rock and 

 producing a pretty mosaic. These veins become darker close to the 

 metamorphosed rock, a certain amount of absorption taking place. 

 The intrusive rock (3918) consists of a micropegrnatitic growth of 

 microperthite and quartz. Tourmaline and chlorite are found, the 

 latter derivative after biotite. The former is but sparingly deve- 

 loped, usually in irregular crystals with bluish pleochroic colours. 

 Microperthite is very common in these rocks, and shows beautiful 

 intergrowths. Patches of felspar with albite-lamellation arc scat- 

 tered through a large mass of orthoclase. Occasionally the inter- 

 growth gives an effect very similar to the cross-hatching of microcliue. 

 The orthoclase is usually the more turbid mineral (3879). 



Another type of acid intrusive is seen in Slide 3919. It is a 

 typical quartz-porphyry intrusive in banded ash. As 

 the result of metamorphism, a light-green fibrous mineral with high 

 polarization-tints has been developed in the altered ash, and is 

 especially abundant at the junction. This mineral (which may be a 

 fibrous hornblende) has been absorbed by the quartz-porphyry, 

 and oecurs throughout the slide. 



