OLDER PERIDOTITES OF 8C0TLA^^D. 



359 



parallel to those displayed by the granites, and so well described by 

 Mr. J. A. Phillips, F.E.S.* Still more frequently the so-called 

 " segregation " veins of peridotite are seen traversing the gabbro, 

 and similar veins of gabbro are found intersecting the peridotite 

 masses. IS^ot unfrequently, too, such veins are found composed of 

 picrite, eucrite, or troctolite. That these so-called "segregation" 

 or " contemporaneous " veins have been formed at different times is 

 shown by the fact that one vein is frequently intersected and shifted 

 in position by another. A beautiful example of this kind is illus- 

 trated in the annexed diagram. 



Intersecting Veins of Gabbro and Dunite in Olivine EocJc, ^ nat . size 



a. Olivine rock (dunite). 



b. Olivine-gabbro vein traversing the same. 



c. Segregation-nodule of felspathie rock. 



d. Porphyritic dunite intersecting and shifting the gabbro-vein. 



As is not uncommonly the case with such veins, the minerals in 

 them have sometimes a special arrangement of their own. Thus in 

 the case figured above, the olivine-gabbro -vein (6) has its felspar 

 grouped principally along the sides, and the augite and olivine in the 

 centre. 



The relations of the gabbros and peridotites in the Western Isles 

 of Scotland seem to indicate that in the heart of these old volcanoes 

 the felspar, olivine, and augite tended to segregate in certaia cases 

 into masses of various dimensions ; and that these masses were, after 

 consolidation, fissured again and again, the fissures being injected by 

 different portions of the magma, which were still in a more or less 

 plastic condition. 



The very close association of peridotites with gabbros is not by 

 any means peculiar to the district we are describing. In the Hartz, 

 in ITassau, in Baden, in Silesia, and in many other districts a similar 

 association of rocks prevails : and in many, if not all of these cases. 



* Quart. Journ. Geol. Soc. vol. xxxvi. (1880) p. 1. 



2*c2 



