Vol. 60.] ROCKS OF THE BORROWDALE VOLCANIC SERIES. 7L 



The Volcanic Series runs in a broad belt, 12 or 13 miles 

 wide, across Cumberland and Westmorland. In order to become 

 acquainted with the various features of the garnetiferous rocks, I 

 have attempted to cover some 130 square miles of ground ; and as 

 the greater part of this area has only been visited once, I feel some 

 diffidence in drawing any conclusions from so hurried a survey. 



Perhaps the most interesting of these garnet-bearing rocks are 

 those which occur as dykes and sills intrusive in the Volcanic Series. 

 They are chiefly to be found in the central part of the district, in 

 connection with the large intrusive masses of the Eskdale Granite 

 and the En nerd ale Granophyre. 



The most typical of these minor intrusions is to be seen in the 

 Langstrath Valley (75° N.W.) at and around Blea Crag. Clifton 

 Ward mapped this as a small laccolite : if such be the case, it is of 

 very irregular form, for on the west side of the valley the branches 

 often run at right angles to the strike of the surrounding rocks. 

 Ward described the rock as a diabase with felsite-veins ; there 

 seems, however, to be an almost infinite variety of rock, from a very 

 fine-grained black, through a coarse porphyritic dark-green rock, to 

 one containing quartz and pink felspar. 



I made an attempt to put in the dividing-lines between these 

 various types, but found that, except for small areas, this was im- 

 possible — the rocks varying in composition and texture every few 

 yards, and shading gradually one into the other. 



Some very good junctions are, however, seen in a small exposure 

 north-north-west of Blea Crag, and separated from it by two small 

 streams. This little section was visited by the members of the 

 Geologists' Association in August 1900. The lines of the junc- 

 tion are here seen to be very sharp — the fine-grained black rock 

 weathering with a smooth, and the coarse rock with a pitted 

 surface. Where the broken lines occur in fig. 1 (p. 72) no distinct 

 junction is seen, but the two types shade one into the other and 

 give rise to a dark-green porphyritic rock. Near the junction of 

 the two the coarse pink rock is found with greenish aggregates, 

 representing, no doubt, portions of the fine-grained rock which 

 have been absorbed and have become more crystalline in the 

 process. It is possible to obtain a perfectly-gradual transition 

 from the fine-grained black rock, through a dark-green to the 

 coarse pink rock, in a single hand-specimen. 



Good junctions are also seen below Blea Crag itself, where the 

 fine-grained rock is plastered against the coarse, and is penetrated 

 by veins of the latter. The junction may be followed up the 

 southern face of the Crag, but it is lost above. 



From these sections it is evident that the fine-grained rock was 

 first intruded, followed by the more acid rock. The interval 

 between the two periods of intrusion must have been short, for the 

 two types have intermixed to form an intermediate one. We 

 have here, then, a case of the intrusion of the basic and acid portions 

 of an already-differentiated magma, and by the intermixture of the 

 two extremes intermediate types have been produced. In the face 



