76 MR. E. E. WALKER ON THE GARNET-BEARING [Feb. IQO4, 



with, cutting through rocks of every kind. A section shows this to 

 be an epidosite, or aggregate of quartz and epidote-grains. These 

 veins are found very commonly along slickensided surfaces, and 

 often give rise to considerable alteration in the adjacent rock, which 

 becomes lighter in colour, a result due no doubt to the infiltration 

 of quartz and epidote. It seems fairly certain that water con- 

 taining substances in solution and at a high temperature percolated 

 through these rocks along lines of weakness. 



Occasionally the weathered surface of these rocks shows a series 

 of parallel * streaky ' lines : these are seen to be due to the infiltration 

 of epidote, etc. along minute planes. One specimen which I obtained 

 showed a slickensided surface roughly parallel to the weathered 

 surface of the rock ; inclined to both occurred a number of fault- 

 planes, rendered conspicuous by infiltrated epidote. We have in 

 this hand-specimen a type of structure which illustrates exceed- 

 ingly well the structure of the whole of the Lake District. The 

 slickensided surface represents the great thrust-plane, the ' streaky ' 

 lines the traces of the lag-faults inclined to the thrust-plane. This 

 specimen is in Mr. Marr's possession. 



Again, a reticulated pattern of crossing lines producing rhombic 

 areas may be seen on the rock-face below Bull Crag. The xeno- 

 liths which occur in the rock have their length along the diagonals. 

 This structure has been produced by movement along two planes 

 almost at right angles one to the other. 



The dykes of the intrusive mass seen south of Sergeant's Crag 

 are continued (according to Ward) on the east side of the Greenup 

 Yalley, and extend for some distance over Ullscarf towards 

 Thirlmere. It is very probable that the dykes mapped in the 

 neighbourhood of Harrop Tarn are closely associated with the 

 Blea-Crag rocks. Numerous basic dykes are mapped near the 

 junction of Angle-Tarn and Allencrags Gills, at the head of 

 the Langstrath. These bear a very strong resemblance to the more 

 basic Blea-Crag rocks, and need no further description. Farther 

 south, a large felsite-dyke is mapped on the Tongue between the 

 two gills. This comprises rocks varying from a dark-green, basic, 

 rather fine-grained type, through coarser rocks of a lighter colour, 

 to a pinkish granophyre, each of which has its representative among 

 the Blea-Crag rocks. 



II. Intrusions in the Bow-Fell District. 



Numerous basic dykes are mapped on the ground north of Bow 

 Fell. One of these runs north-north-west and south-south-east for 

 some distance, and shows interesting variations of rock. All types 

 are represented, from a dark-green fine-grained rock, with garnets 

 and a porphyritic ferromagnesian mineral, to a coarse rock with 

 garnets and pink-white felspars (3849-3853). The ferromagnesian 

 mineral is augite in colourless crystals, showing lamellar twinning 

 parallel to 100. The angle between the twinning-line and the 

 cleavage varies from 26° to 43°. The felspar is an oligoclase- 

 andesine with edges obscured by dirty matter, and often replaced by 



