294 MISSUS. A. HARKER AND J. E. MARR ON 



character of the rock, and compares closely with the figures found 

 for other Lake District andosites. Two rocks from Mr. Clifton 

 Ward's Falcon Crag section (near Keswick) gave 60*718 and 

 59-511 per cent., and one from Lingmell Beck, about two miles 

 north-west of Scawfell Pike, 59-151 per cent. To these we may 

 add a large boulder at Manfield, near Darlington, doubtless from the 

 Lake District, in which Mr. W. F. K. Stock found 59*87 per cent, 

 of silica *. 



The andesitic lavas afford some of the most beautiful and instruc- 

 tive examples of thermo-metamorphism in the district, and as they 

 can be followed along their line of strike from localities free from 

 alteration into the "contact-aureole" and up to their junction with 

 the granite itself, the process of transformation can be traced in all 

 its stages. 



At places at considerable distances from the junction, such as 

 Little Saddle Crag (1350 yards), we notice that the vesicles contain 

 a quantity of epidote in addition to delessite, quartz, and calcite 

 [1277, 1278]. In some districts epidote has been recorded as a 

 product of thermal metamorphism, but we cannot satisfy ourselves 

 that in these andesites it is other than an ordinary result of 

 weathering action. Setting it aside, the rocks at the locality in 

 question give no marked indication of metamorphism by the granite. 

 Proceeding eastward, however, we find substantial alteration setting 

 in, being first shown in the weathering-products of the original 

 rock. A specimen taken some distance west of Wasdale Pike, and 

 nearly 800 yards from the granite-boundary, is crowded with 

 minute flakes of brown mica, apparently developed at the expense 

 of the decomposition-product (delessite ?) disseminated through the 

 weathered andesite [1205]. In the vesicles part of the delessite is 

 altered into green hornblende, while part remains unchanged (see 

 PI. XI. fig. 4). A little farther east (at 750 yards) the flakes of 

 brown mica are rather larger and more collected [1204J. The pale 

 green product in the vesicles is still only partially altered, and a 

 little epidote is still present, but this mineral is not found nearer to 

 the granite. 



The formation of brown mica, which, it will be seen, is the most 

 characteristic mineral in the altered andesites, is thus the first clear 

 result of the metamorphic action, while, almost concurrently with 

 it, green hornblende begins to appear among the contents of the 

 vesicles. The greatest distance at which wo have verified meta- 

 morphic action is 1150 or 1200 yards, on Low Fell, where the 

 mica-flakes are chiefly collected about little grains of magnetite, a 

 frequent occurrence in such rocks [1279]. 



In the field the early stages of metamorphism are indicated 

 chiefly by the vesicles, in which lustrous, greenish-black aggregates 

 of hornblende are to be detected by the eye. Proceeding towards 

 the contact, the hornblende becomes more distinctive, showing good 

 cleavage-planes, while the quartz filling other vesicles takes on a 



* 'Naturalist' (1880), p. 304. 



