Vol. 50.] AND OTHER ASSOCIATED ROCKS AT BARNTON. 41 



among the smaller grains, the -whole or nearly the whole grain has 

 been changed into the green mineral. The extinction-angle of these 

 minerals may be a little oblique, but of that I am not quite sure. 

 In any case there can be no doubt that they aro alteration-products 

 of olivine. Most of the grains contain scattered specks of iron 

 oxide. 



(2) Augite of a light pink colour, in grains of most irregular 

 shapes, fairly well preserved and sometimes more than yL inch in 

 length. The larger grains contain enclosures of the altered olivine, 

 microliths, etc. 



(3) Felspar. A very few lath-shaped crystals : the largest mea- 

 sures 0-04 x 0-0075 inch. 



(4) Mica in flakes, of which one of an average size measures 

 - 015x 0-01 inch. They are orange and yellow-brown in colour, 

 and are dichroic. 



(5) Iron Oxide, in grains and granules down to fine dust, 

 scattered through the altered olivine. It does not occur in the 

 augite, except in connexion with altered olivine-enclosures. 



There is in places much interstitial matter of a turbid white 

 appearance, which may be, at least in part, altered felspar. 



In pi. xxvii. of the 'Mineralogie Micrographique ' of MM. Fouque 

 and Michel-Levy a micro-section is figured of a rock in which 

 alteration has taken place, somewhat like the alteration of the 

 olivine in the Barnton picrite. The crystals in the Trench rock 

 are, however, more regular and clearly defined than the crystals of 

 altered olivine in the Barnton rock— which are not unlike the 

 serpentinous pseudomorphs after olivine in the micro-section of the 

 Menheniot picrite figured by Mr. Teall. 1 



In one illustration of the Incheolm picrite by the same author 2 

 a crystal is shown, surrounded by small flakes of hornblende, which 

 is not unlike the green crystals in bed No. 6 of the section here 

 described, in which, however, the peripheral mineral is brown 

 mica. It will be remembered that the island of Incheolm lies in 

 the Firth of Forth, 4| miles north of the Barnton railway-cutting. 



I submitted a micro-section from the Barnton picrite to Prof. 

 Bonney, F.R.S., and he has very kindly furnished me with the fol- 

 lowing remarks on its affinities with other picrites : — "The change 

 of the olivine here exhibited is not the one so usually seen. It is 

 more regular, more uniform, and more analogous to the change of 

 the enstatite-group. We have it in the Porthlisky picrite, the 

 Menheniot picrite (but here are some other peculiarities in the net- 

 work of microliths), traces of a similar structure in the Duporth 

 rock (here, however, I suspect an altered bastite), also in a picrite 

 boulder from near Liskeard (possibly connected originally with 

 Menheniot). I have, however, seen it in some true serpentines. 



" The Barnton rock comes in many respects very near to the 



1 ' Brit. Petrog:r.,' 1888, pi. ii. fig. 2. 



2 Ibid. pi. vii. fig. 1. 



