OLDER PERIDOTITES OF SCOTLAND. 373 



result of alteration, and is not found in perfectly fresh forms of the 

 mineral. In some cases I have only heen able to verify the presence 

 of enstatite by a micro-chemical method. In thin slices and in the 

 powder of the rock the olivine is attacked by long digestion in strong 

 hydrochloric acid, while the enstatite remains unchanged. 



In other districts of the Western Isles of Scotland we find more 

 ferriferous varieties of enstatite which must be referred to proto- 

 bronzite and proto-hypersthene. Usually, these are in a more or less 

 altered condition. In very few cases, so far as I have seen, is 

 the enstatite the principal pyroxenic constituent of the rock, but it 

 is subordinate to the augite. In small patches of these rocks, how- 

 ever, the augite ma}- become subordinate to the enstatite, while 

 occasionally the former mineral may be altogether wanting, the rock 

 thus passing into a norite or hjpersthenite. 



Occasionally the dark colour of the mineral, when seen in the 

 thinnest sections by transmitted light, and its strikingly vivid pleo- 

 chroism, indicate that we are dealing with the excessively pleochroic 

 enstatite, amblystegite (see PL XI. fig. 7). That Macculloch was 

 acquainted with the altered forms of the enstatite in these rocks, 

 I have the clearest evidence. A specimen given by Macculloch 

 to the late Mr. Majendie passed into the hands of Mr. Warington 

 Smyth, and by him was placed in the collection of the IN^ormal 

 School of Science and Royal School of Mines. An examination of 

 the optical properties of this specimen shows it to be a rhombic 

 and not a monoclinic pyroxene. 



Intergrowths of the monoclinic and rhombic pyroxenes occur not 

 unfrequently in the gabbros and peridotites of the Western Isles of 

 Scotland, and crystals of the one mineral are sometimes found 

 enclosed in the other. I have not, however, detected examples of 

 the twin-intergrowths of the two minerals described by Trippke *. 



The Olivines of the gabbros and peridotites of the Western Isles 

 of Scotland appear, like the associated augites and enstatites,to belong, 

 in some cases, to the most highly ferriferous varieties. Seen in 

 thin sections under the microscope, these olivines are often found 

 to be by no means colourless, like most of the olivine of basalts, 

 but exhibit a yellow tint similar to, but not so intense as, the 

 tint of the fayalite in eulysite. Such deep-coloured olivines are 

 abundant in the Shiant Isles. 



Some of the olivines of these rocks are perfectly fresh and un- 

 weathered, but occasionally the mineral shows incipient traces of 

 scrpentinization along its cracks. Occasionally another kind of 

 change has taken place resulting in the mineral acquiring a yellow 

 tint along its fissures, which causes it to assume the colour and 

 aspect of chondrodite. Similarly altered forms of olivine from the 

 Kaiserstiihl have received the names of "^ chusite " and " limbelite *'. 



An analysis of this curiously altered olivine from the summit of 

 Halival in the island of Hum, was made by Dr. Heddle and is as 

 follows t (its specific gravity was found to be 3-327) : — 



* Neues Jahrb. fiir Min. &c. 1878, p. 673. 

 t Mineralogical Magazine, vol. v. (1884) p. 16. 

 Q. J. G. S. No. 163. 2 d 



