392 PROF. J. W. JUDD ON" THE TERTIARY AND 



with the several minerals sometimes disposed in bands parallel to 

 the sides of the vein. (See p. 359.) 



One of the varieties of peridotite in the island of Knm is a rock 

 of such beauty as to have attracted the attention and excited the 

 admiration of all visitors to the island *. It is found constituting 

 considerable portions of the mountains of Halival, Haiskeval, and 

 Tralival, and is seen passing everywhere into an augite-gabbro. 

 Different portions of these great mountain-masses appear to vary 

 chiefly in the quantity of felspar which they contain. 



The felspathic varieties are true augite-gabbros, consisting of a 

 very fresh felspar, often perfectly clear and glassy in appearance, a 

 bright green augite, and an olivine which has undergone the peculiar 

 alteration which makes it resemble in aspect chondrodite. The 

 rock is sometimes fine-grained and at others very coarse-grained, 

 and the mixture of colourless, bright-green, and yellow crystals is 

 very striking. 



The non-felspathic varieties are picrites and Iherzolites, rocks 

 having a specific gravity of about 3-20, the admixture of bright- 

 green augite, and the yellow olivine with more or less enstatite, 

 giving them a very beautiful appearance (see PI. XTII. tig. 3). 



The augite of these rocks is usually of a bright emerald-green 

 tint by reflected light, and pale green passing into pale brown by 

 transmitted light. It usually exhibits a very feeble pleochroism. 

 The augite sometimes forms well- developed crystals, but more 

 usually it exists as rounded grains, like those of coccoiite. The 

 composition of this diopside, or slightly ferriferous augite, is illus- 

 trated by the analysis quoted on p. 367. This green augite, as 

 well as the pale brownish varieties which accompany it, is traversed 

 in all directions by cracks which are marked by numerous enclosures, 

 which are sometimes liquid- or gas-cavities, but are not unfre- 

 quently filled with dark-brown or black solid materials. These 

 augites exhibit admirable illustration of the initial stage of Schil- 

 lerization. A few scattered tabular enclosures make their ap- 

 pearance along planes parallel with the orthopinacoid, and these in 

 other examples are seen multiplied until the augite becomes a 

 typical diallage. 



The olivines of these rocks form more or less rounded or oval 

 grains, often enclosing globular particles of chromite or picotite, 

 which are generally black or opaque at their centres, but slightly 

 translucent, and dark-brown in colour at their edges. A very 

 marked feature of these olivines is their yellowish or brownish- 

 yellow tint, so different from that of the mineral in its normal 

 condition. Under the microscope this peculiar colour is seen to be 

 confined to the cracks which traverse the crystals in all directions. 

 The yellow tint is present in cases where not the smallest trace of 

 serpentinization can be detected in the crystal. Along these cracks 

 we find the curious stellate enclosures being developed, and these 



* See Jameson, 'Mineralogy of the Scottish Isles,' vol. ii. p. 51 ; MaccuUocli, 

 ' The Western Isles of Scotland,' vol. i. p, 485 ; Heddle, Trans. Eoy. Soc. Edin. 

 vol. xxviii. (1879) p. 478. 



