OLDER PERIDOTITES OF SCOTLAND. 393 



may multiply till they form black bands, traversing the crystal in 

 all directions ; lastly similar inclusions, sometimes mingled with 

 brown plates, make their appearance along a series of parallel planes 

 traversing the crystal in the direction of the optic axis, and these 

 increase in number till they communicate a dusty appearance to the 

 crystal. 



The accessory minerals of these peridotites of Halival and the 

 adjoining mountains of Kum are as follows : — felspar, a clear 

 variety, crystallizing in foi ms intermediate between the lath-shaped 

 crystals of basalt and the broad forms common in the massive 

 gabbros, is nearly always present in small quantities, and may 

 increase in abundance till the rock passes into a gabbro ; fer- 

 riferous enstatite (hypersthene), which, when it occurs with the 

 granular variety of augite, also assumes similar granular forms, but 

 is at once distinguished by its colour, its remarkable pleochroism, and 

 its extinction in positions parallel to the vibration-planesof the crossed 

 nicols ; biotite occurs but rarely, while magnetite and chromite 

 (or picotite) are universally present. It appears as though the 

 opaque magnetite passes by insensible gradations into the trans- 

 lucent and deep-brown chromite or picotite. 



The last, but by no means the least interesting, of these Tertiary 

 peridotites of Scotland which I shall notice, is that which occurs in 

 the Shiant Isles to the North of Skye ; it exhibits the most beautiful 

 example of the ophitic structure in these rocks. 



The igneous rocks of these islands, as pointed out in my previous 

 paper*, form a great intrusive sheet 500 feet in thickness forced 

 between strata of Inferior- Oolite age. The vertical columns of this 

 intrusive mass constitute, as Macculloch has pointed out, one of the 

 most imposing spectacles in the British Islands. The columns, 500 

 feet long and from 4 to 6 feet in diameter, form a range of inaccessible 

 precipices rising directly from the sea. 



Owing to the rising of a storm, I was compelled to render my 

 visit to these little-known islands shorter than I could have wished, 

 and consequently I had an opportunity only of tracing the relations 

 of the igneous to the sedimentary rock-masses, and of collecting the 

 fossils from the latter. I was not able to devote any time to deter- 

 mining the relations of the different varieties of igneous rocks to 

 one another. As many varieties of the rocks as possible were, 

 hovTOver, collected from different parts of the island by myself and 

 a friend. Dr. Taylor Smith f, who accompanied me. 



The rock of this great intrusive sheet was classed by Macculloch 

 as an " augite-rock,"+ the augite being by far the most conspicuous 

 mineral in it. A careful study of the large series of specimens 

 brought from the islands shows that about one half of them should 

 be classed as ophitic dolerites, and the other half as peridotites. 

 But every specimen collected shows such variations owing to the 



* Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 677. 



t I am greatly indebted to Dr. Taylor Smith for allowing me to study the 

 whole of his specimens in connexion with my own. 

 \ Western Isles of Scotland, vol. i. p. 439. 



