514 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JimO 6, 



nication was traversed by Sir Eoderick Mnrchison, accompanied by 

 Professor J. Nicol, in 1850 ; and the former geologist subsequently 

 published the results of that journey, in a memoir upon the geology 

 of the South-western Highlands of Scotland* ; but as the end Sir 

 Eoderick had in view was mainly to ascertain the structure of the 

 strata and determine their position, his allusions to the metamorphic 

 rocks are necessarily brief. In the spring of 1865 the Geological 

 Survey was extended into the southern regions of Ayrshire, and thus 

 afforded my colleagues (Dr. Young and Mr. A. Geikie) and myself 

 ample facilities for a more minute examination of the beds in Cjues- 

 tion. "We recognized the metamorphic character of certain diorites, 

 serpentines, and crystalline felspathic rocks, independently of each 

 other ; and in the area that fell to me to survey I was enabled to 

 trace passages between the various altered rocks, which seem to 

 throw light upon the obscure process of metamorphic action. The 

 interest which is attached to the altered strata of this district chiefly 

 consists in the fact that they exhibit certain arrested stages of 

 metamor]3hic action, so that we can in many instances begin with 

 unaltered beds and trace the gradual changes which they undergo 

 during their passage into crystalline and pseudo -igneous rocks. The 

 large masses of amygdaloid indicated upon the sketch map which 

 accompanies Sir Eoderick's memoir, notwithstanding their exceed- 

 ingly igneous aspect, are nevertheless, as will be shown, not of 

 igneous but of metamorphic origin. Of the same nature are the 

 porphyries, diorites, and serpentines. The resemblance to trap 

 which many of those rocks exhibit is so striking that it will per- 

 haps be difficult to convince a geologist who has not minutely 

 examined the ground for himself of their true character. 



"With Sir Eoderick Murchison's permission, some of the more 

 interesting results obtained during the progress of the Survey are 

 here described. 



The metamorphic Lower Silurian rocks surveyed by us may 

 be roughly estimated to cover an area of forty-five square miles 

 or thereabouts. They extend in the direction of the strike (i. e. 

 N.E. and S.W.) for ten miles or so, with an average breadth 

 of three miles and a half; but several isolated portions occur 

 beyond the main mass of metamorphic strata. Beginning a 

 little to the south of Girvan, the altered rocks continue, with 

 little interruption, along the coast-line as far as Currarie Port 

 — a distance of about twelve miles. As the coast-line cuts the 

 strata at an acute angle to their strike, nearly all the varieties of 

 metamorphism characteristic of the district are met with in this 

 section. The northern limits of the metamorphic area are exceed- 

 ingly irregular. The lines traced by us in that direction show altered 

 beds extended among unaltered deposits ; and the boundary is still 

 further confused by faultings, and by overlaps of Old Eed Sand- 

 stone. Towards the south, however, the metamorphic strata are 

 separated from the unaltered greywackes by a long wavy fault which 

 runs inland from the sea- coast at Glendrishaig, and, keeping to the 

 * Quart. Journ. Geol. Soc, vol. vii. p. 137 his. 



