770 T, G. BONNEY ON THE SERPENTINE AND 



expressed grave doubts as to the correctness of Dr. Geikie's reading 

 of the petrography of the district. One of the serpentines among 

 these specimens could not have been distinguished from a variety 

 from the Lizard. Accordingly I determined to examine them as 

 soon as I had finished my Cornish work. This I did in July 1877, 

 and had the advantage of being accompanied by my friend the Rev. 

 E. Hill, who had been my companion during my last visit to the 

 Lizard. I am allowed to say that he quite concurs with my interpre- 

 tations of the sections. On returning home I had a series of slides 

 prepared from the more important specimens which I had collected, 

 and some chemical analyses have been kindly made for me by P. T. 

 S. Houghton, Esq., B.A., Scholar of St. John's College. 



We drove from Grirvan to Colmonell by the coast road as far as 

 Lendalfoot; the result of this journey and of an afternoon's walk 

 inland was to convince me that here, as in Cornwall, the relations 

 of the rocks would be most clearly demonstrated on the sea-shore, 

 cultivation and vegetation making it almost hopeless to obtain good 

 sections inland. We accordingly worked the coast carefully from 

 Balcreuchan Port to Pinbain Hill (about 4| miles), the only part 

 where the serpentine reaches the shore. The result has been that I 

 find myself, as will appear, in several important respects, quite 

 unable to accept Dr. Geikie's interpretation of the petrography of 

 this part of Ayrshire. 



Serpentine of Balhamie Hill. 



Before proceeding to the shore- sections I will briefly describe the 

 serpentine of Balhamie Hill, near Colmonell, which can be well 

 examined in a pit near the roadside, about three quarters of a mile 

 from the village. This rock can hardly be distinguished from the 

 black serpentine north of Cadgwith, Cornwall*. It has exactly the 

 same sharply defined but rather irregular rhomboidal jointing. The 

 joints are coated with greenish or whitish steatite, often show 

 slickensides, and turn brown after exposure to the weather. The 

 rock itself, on old surfaces, assumes the same rugged grey-brown 

 aspect as that at the Lizard ; its fracture is subconchoidal. The 

 ground-mass is a deep black, weathering to a pale grey-green. It 

 is full of crystals of glittering bronzite f , just like those in the Cadg- 

 with rock. A detailed description of the microscopic structure is 

 needless ; for what I have written of that will apply here almost 

 word for word, the only difference being that, in the specimen 

 examined, the conversion of the olivine into serpentine is complete. 

 The " strings " of doubly refracting serpentine, stained and clothed 

 with granules of black ferric oxide, exhibit a rude parallelism over 

 part of the slide. The bronzite also in this rock is a little more 

 altered than it is at Cadgwith ; some grains contain numerous fer- 



* Quart. Journ. G-eol. Soe. xxxiii. pp. 899, 921, 925. 



t I mean by bronzite the variety of enstatite which has a metallic lustre. In 

 my paper on the Lizard I avoided the former term, as it had often been used 

 indefinitely by English authors, and I wished to call special attention to the fact 

 that the Cornish mineral was not diallage. 



