Vol. 60.] ROCKS OF THE BORROWDALE VOLCANIC SERIES. 93 



ravine. A similar mosaic occurs in Sourmilk Gill, opposite Sea- 

 thwaite Farm, where the rocks round the graphite-lode arc intrusive 

 into banded ash. 



It would be impossible to conclude from the sections at Buscoe 

 Tarns and on Aaron Crags, Seathwaite Fell, that the garnet-rock 

 is intrusive. The appearances would be better explained on the 

 supposition that the flinty ash was the intrusive rock. This 

 intermingling at the junction between the two rocks has been 

 caused by intense pressure. The bedding-plane between the 

 soft Banded Ashes and the harder garnet-bearing rocks has 

 very probably been one of lag-faulting ; the pressure has been 

 so great that the ash has been altered and incorporated with the 

 garnet-rock. 



An excellent section about 1500 feet up, almost due west of the 

 Langstrath Gorge, illustrates this action. To the north the streaks 

 are seen to dip southward, at an angle varying between 2Q Z and 30 : . 

 As the junction with the banded ash is approached, the ' streaky ' 

 lines become horizontal. They develop into whorls, and small thrust- 

 planes can be distinctly seen. Fragments of ash become incorporated 

 close to the junction. If the garnet-rocks were intrusive, we should 

 expect to find some evidence of intrusion at the lower junction. Here 

 a complex is by no means common, and where it is absent no meta- 

 morphism has taken place. 



A perfectly-gradual passage from ash to ' streaky ' rock is seen 

 on the crags west of Galleny Force, IStouethwaite. Elsewhere on 

 Kosthwaite Fell disturbance has takeu place, and we have altered 

 flinty ash running in thin veins at right angles to the bedding. It 

 would appear that the pressure was so great that the ash was 

 either partly melted, or became sufficiently plastic to be forced into 

 the surrounding rock. 



The junction on the south-eastern slope of Sourmilk-Gill Combe 

 affords evidence of great pressure. The rock at the junction 

 is of a dark green, with bands of lighter green representing 

 the incorporated ash. This rock is so much hardened that the 

 softer ash has weathered away from beneath it, thus leaving a 

 ledge of rock which projects 2 or 3 feet from the hillside. 



This evidence of the operation of pressure in the production of 

 these friction-breccias, combined with the obviously-fragmental 

 nature of the rocks, serves to dispose of the intrusive theory. The 

 reason of the non-occurrence of the 'streaky' rocks in the south has 

 yet to be explained. 



Many of these ' streaky ' rocks contain derived fragments, and 

 these may be lenticular or of irregular shape. A glance at the 

 weathered surface would, in many cases, convey the idea that the 

 rocks were fragmental ; in others, that they were lavas showing 

 flow-structure. Included fragments in lavas, especially in the 

 rhyolites, are quite common. The intrusive rocks of the district 

 are, moreover, full of xenoliths, so that there seems to be no 

 insuperable difficulty in accepting the third explanation of the 

 4 streaky ' character. 



