CHAP. VI 
VOLCANIC BOSSES 
95 
Lower Silurian lavas of the Lake District, the enveloped portions of the 
latter show considerable modification. Their groundinass becomes darker 
and more lustrous, the felspars assume a clearer appearance and lose some 
of their conspicuous inclusions, the pyroxenie constituents are converted 
into pale amphihole, and the glassy base disappears. At the actual line of 
contact the felspars of the lavas have become disengaged from their original 
matrix, which seems to have been dissolved and absorbed in the gabbro- 
magma. Brown mica has been exceptionally developed in the altered lava. 
At the same time, a change is noticeable in the character of the gabbro 
itself near the contact. Brown mica is there to be seen, though not a 
constituent of the rock elsewhere. The eruptive material has incorporated 
the basic groundinass of the lavas, leaving the felspars uudissolved.^ 
Much more serious are the changes produced by intrusions of acid 
material, though here again the metamorphism varies within wide limits, 
being sometimes hardly perceptible, and in other cases advancing so far as 
to convert mere sedimentary material into thoroughly crystalline rocks. 
Small sills and dykes of felsite and granophyre may produce very slight 
change even upon shales and limestones, as may be seen among the erup- 
tive rocks of Skye and Eaasay. Large bosses of granophyre, and still more 
of granite, have been accompanied with the most extensive metamorphism. 
Bound these eruptive masses every gradation may be traced among sandy 
and argillaceous sediments, until they pass into ■ crystalline mica -schists, 
which do not appear to be distinguishable from rocks of Arcluean age. 
Admirable examples of this extreme alteration may be observed around the 
great granite bosses of Galloway.^ Again, among calcareous rocks a transi- 
tion may be traced from dull grey ordinary fossiliferous limestones and 
dolomites into pure white crystalline marbles, full of crystals of treinolite, 
zoisite, garnet and other minerals. The alteration of the fossiliferous Cam- 
brian limestones of Strath in Skye by tlie intrusive bosses of Tertiary granite 
well illustrates this change.'* 
Without entering further here into the wide subject of contact meta- 
morphism, to which a large literature has now been devoted, we may note 
the effects whieli have been produced in the eruptive material itself by its 
contact with the surrounding rocks. Not only have these rocks been 
altered, but very considerable modifications have likewise taken place in the 
active agent of the change. 
Sometimes the alteration of the invading material has been effected 
without any sensible absorption of the mineral constituents of the rocks 
invaded. This appears to be the case in those instances where sheets of 
basalt, intruded among coals or highly carbonaceous shales, have lost their 
compact crystalline character and have become mere clays. In the coal- 
Mr. Harker, Quart. Joxmi. (h'.ol. Soc. vol. 1. (1894), p. 331. 
2 See Explanation to Sheet 9 of the Gcoloxfical Survey of Scotland, p. 22 ; Prof. Bonney and 
Mr. Allport, Proc. Roy. Soc. xvi. (1889) ; Miss Gardiner, Quai-t. Jouru. Geol. Soc. vol. xlvi. 
• (1890), p. 569. 
3 Macculloeh, Trans. Geol. Soc. vol. iii. (1816), p. 1; Description of the n'estern Isles, vol i. 
p. 322. See also Quaii. ,/ourn. tlcol. Sue. vol. xiv. (1857), p. 1 ; and vol. xliv. (1888), p. 62. 
