Vol. 60.] ROCKS OP THE BORROWDALE VOLCANIC SERIES. 85 



Buttermere rock I have not seen these garnet-bearing rocks in situ, 

 but the numerous loose blocks in Burtness-Combe Gill afford 

 sufficient testimony to their occurrence. 



V. Other Garnetiferous Intrusions in the Lake District. 



Acid garnet-bearing dykes occur in all parts of the district. The 

 dyke mapped at Fox Tarn on Scawfell is a good quartz- 

 porphyry containing quartz, microperthite, plagioclase, chlorite 

 after biotite, garnet, and apatite. The garnets are surrounded by 

 quartz, but they are broken up and replaced by chlorite and epidote. 

 The occurrence of this latter mineral points to a fair percentage of lime 

 in the garnet. A very similar rock occurs low down in the stream 

 and on the western flank of Rosthwaite Combe, and roughly at the 

 same horizon in a small patch on Rosthwaite Fell. This is a greenish 

 rock, with pink-white felspars and garnet. The latter mineral 

 has usually one or two faces well developed, but is elsewhere 

 embayed by quartz. 



More basic garnetiferous rocks occur at Dock Tarn, Harrop Tarn, 

 and along the Wythburn valley. Many of the rocks mapped as 

 lavas by Ward in Wythburn and on Helvellyn are in all probability 

 intrusive. The Dock-Tarn rock is found east-north-east of Stone- 

 thwaite Church, and runs from the valley to the top of the Great 

 Crag ; the path to the top of the hill follows its outcrop very closely. 

 It may also occur on the lower slopes of Rosthwaite Fell. It was 

 mapped as a lava, but Mr. Marr proved its intrusive nature from 

 the metamorphism of the surrounding ash. It is a minor sill, 

 intrusive into garnet-breccia and banded ash. The upper limit is 

 well defined by the altered ash, but the lower limit is hard to make 

 out, for it is somewhat difficult to distinguish between cleaved 

 intrusive rock and the garnet-breccias. It may be described as a 

 sill of garnetiferous andesite, with phenocrysts of 

 plagioclase. The rock is in parts intensely cleaved, the felspars 

 become broken up, and the garnets are altered to chlorite and white 

 mica with separation of iron-ore. Flakes of chlorite and sericite are 

 developed throughout the rock. The percentage of silica is 58*08. 



Similar rocks are found at the base of the E3*cott Lavas, imme- 

 diately south of the stream issuing from Dock Tarn, where the 

 same kind of metamorphism occurs ; and also in the triangle formed 

 by the roads at the western end of the Thirlmere dam. Here the 

 metamorphosed ash only becomes white and porcellanized close to 

 the junction, no new minerals being developed. 



A quarter of a mile north-west of Harrop Tarn, Ward mapped a 

 massive lava : this Mr. Marr considered to be intrusive. The 

 microscopical characters of the rock bear out this supposition. 

 It is a quartz-garnet porphyrite, with highly-altered plagio- 

 clase. Quartz occurs in rounded and corroded crystals, surrounded 

 by a zone of lighter groundmass. Pseudomorphs of chlorite, epidote, 

 and calcite after an original ferrorn agues ian mineral, apatite, and 



