390 PROP. J. W. JUDD ON THE TERTIARY AND 



and its flame-reaction, is proved to be anorthite) and olivine. I 

 was at a loss to find any exact analogue of these felspar-olivine 

 rocks ; but Professor Bonney has pointed out to me that in some 

 varieties of the forellenstein (troctolite) of Yolpersdorf, in Silesia, 

 the augite becomes inconspicuous and almost disappears, and we 

 have a rock similar to the Scotch variety which I have indicated. 

 Professor Bonney has found the same rock in an altered state at 

 Coverack Cove in Cornwall*. Other veins and enclosures in these 

 rocks consist of anorthite and augite, and may be classed with the 

 eucrites (see PL XIII. figs. 1, 2, 3, 4, 5, & 6). 



As all of these varieties of the Tertiary ultra-basic rocks are found 

 passing into one another, and into the gabbros and dolerites, by an 

 increase in the quantity of one mineral or by the diminution and 

 disappearance of others, it will, I think, be more instructive to con- 

 sider the varieties exhibited by the rocks of difi'erent localities when 

 their microscopic structures are considered than to dwell upon the 

 merely accidental varieties of mineralogical constitution. 



It may be mentioned at the outset that, as a general rule, the 

 peridotites vary in structure with the gabbros and dolerites with 

 which they are associated. We thus find peridotites of granitic 

 structure, others of granulitic structure, and others, again, of ophitic 

 structure (see PI. XIIL). 



The most perfectly crystalline type of these peridotites is found 

 in the valleys in the central part of the island of Eum. A typical 

 example of these rocks collected near the road between Kinloch and 

 Harris is highly crystalline, of a black colour with a few scattered 

 felspar-crystals, and has a specific gravity of 3-18. The olivine of 

 this rock is of a nearly black colour, owing to the abundance of 

 magnetite enclosures it contains ; it can, however, be distinguished 

 from the augite by its lustre and its fracture. It is not surprising 

 to find that MaccuUoch, misled by the unusual colour of the olivine, 

 failed to distinguish that mineral ; and, regarding the rock as being 

 wholly composed of augite, he gave to it the name of " augite- 

 rock." 



Thin slices under the microscope show the rock to be essentially 

 made up of large crystals of augite and olivine. 



The augite is of pale greenish or brownish tint by transmitted light, 

 and exhibits very faint dichroism ; but its cleavage-cracks, which 

 are well marked, are characteristic of the mineral. Both the 

 cleavage- and irregular cracks of the mineral are marked by bands 

 of enclosures, consisting of cavities, sometimes empty, sometimes 

 containing a liquid with a bubble, but most commonly filled with a 

 solid substance of a reddish brown, sometimes almost black colour. 

 This augite only occasionally exhibits the first traces of a passage 

 into diallage by the development of enclosures in planes parallel to 

 the orthopinacoid. 



The olivine of this rock is in the most beautifully fresh condition, 

 and seldom shows any trace of serpentinization. It polarizes with 

 * Quart. Journ. Geol. Soc. vol. xxxiii. (1877) p. 909. 



