ON THE SERPENTINE ROCKS ETC. OE THE AYRSHIRE COAST. 769 



44. On the Serpentine and associated Igneous Rocks of the Ayrshire 

 Coast. By Rev. T. G. Bonney, M.A., F.R.S., Sec. G.S., Pro- 

 fessor of Geology at University College, London, and Fellow of 

 St. John's College, Cambridge. (Read May 22, 1878.) 



Introductory. 



Serpentine of Balhamie Hill. 



The " Dioritic " Bocks. 



The Serpentines of the Coast. 



Contents. 



The Gabbros. 

 The Porphyrites. 

 Later Basaltic Dykes. 

 Conclusion. 



Introductory. 



In the twenty-second volume of this Journal is a paper by Dr. J. Geikie 

 on the metamorphic Lower Silurian rocks of Carrick, Ayrshire. The 

 author endeavours to prove that the rocks of the district, with some 

 few slight exceptions, are of sedimentary origin, a "felspar por- 

 phyry" being the "maximum stage of metamorphism exhibited by 

 the felspathic rocks of the district." The " dioritic rocks," " diorite 

 and hypersthenite, both of which are occasionally foliated," are also 

 regarded as of metamorphic origin ; and, lastly, the " compact ser- 

 pentines, like the altered strata with which they are associated, are 

 truly bedded rocks." This view is also very clearly enunciated in 

 the ' Descriptive Catalogue of Rock Specimens collected by the Geo- 

 logical Survey of Scotland, and exhibited in the Edinburgh Museum 

 of Science and Art ' (ed. 1870), p. 49 &c, from which I quote a few 

 sentences. " The altered rocks of this district may be grouped into 

 three classes — felspathic rocks, dioritic rocks, and serpentine. The 

 felspathic rocks are by far the most abundant, and vary much in 

 structure and aspect ; the diorites and serpentine have a more definite 

 character, and are often interbedded or closely associated. But the 

 one class shades into the other, and no strongly marked line of 

 division between them can be drawn upon a map. The metamor- 

 phic changes which have affected these Lower Silurian strata can 

 hardly be exhibited in hand specimens, but must be studied in the 

 field." 



The paper referred to above was severely handled on chemical 

 and mineralogical grounds by the late Mr. D. Forbes * ; it is, how- 

 ever, quoted in text-books f as establishing the above conclusions. 

 These, however, Mr. Forbes's criticisms and my own work in various 

 localities have long made me hesitate to accept J; and about two 

 years since my scepticism was increased when Mr. S. Allport showed 

 me some specimens of serpentine from the vicinity of Colmonell, and 



* Geol. Mag. vol. iv. pp. 49 & 225. 



t E. g. Jukes and Geikie, p. 142 ; Green, pp. 292, 295. 



\ The late Sir B. I. Murchison, who described this district (Quart. Journ. 

 Geol. Soc. vii. p. 137), declined, as I find (Siluria, p. 155, ed. 1867), to abandon 

 the idea of the igneous origin of some at least of these rocks. 



Q. J.G.S. No. 136. 3f 



