GABBKOS ETC. TN SCOTLAND AND IRELAND. 85 



normal tint of the augite in a rock and the unequally distributed 

 tints acquired by alteration. 



Sometimes the augites, both the pale-coloured and the brown 

 varieties, are found undergoing paramorphic change into hornblende. 

 In these cases the^rs^ change seems to consist in the acquisition of 

 the characteristic colour and pleochroism of the hornblende. Thus 

 we find the purplish-brown augite of the ophitic dolerite of the 

 Shiant Isles (Plate V. fig. 1) assuming the yeUo wish-brown tint 

 and the strong pleochroism of hornblende, while the cleavage-cracks 

 and other features of the augite still remain. (See ante, p. 60, 

 footnote.) But sometimes a brown augite passes into a green 

 hornblende and vice versa ; and not un frequently crystals of the 

 same augite may be found passing into both green and brown 

 hornblende. This subject has been carefully studied in many of 

 the older rocks by Professor Bonney, F.H.S. * When the augite 

 has by schiUerization been converted into diaUage, the changes 

 produced by superficial action may be somewhat modified. The 

 augite, from which much of the iron has been separated, shows 

 a great tendency to pass into the bright green actinolitic varieties 

 known as smaragdite. So great is this tendency that there are 

 cases in which gabbros, with olivines only slightly serpentinized, 

 have their diallage-crystals bordered by fringes of smaragdite. The 

 first stage of this change, which is accompanied by the separation 

 of calcic carbonate and other decomposition-products along the 

 planes of foliation of the diallage, leads to the formation of the 

 beautiful varieties known as '•' green diallage." 



But in other cases the change of the augite takes place in an 

 entirely different manner. The whole mass of each augite-crystal 

 is seen passing into the green structureless material known as 

 " viridite ;" incipient crystallization in this viridite not unfrequently 

 leads to the formation of vermiform aggregates like those so often seen 

 on a larger scale in the minerals of the chlorite group. Eventually, 

 however, the who e of the viridite substance may acquire the 

 pleochroism and cleavage of hornblende. As the product of this kind 

 of change is uralite, the change may be called " uralitization." 

 There are thus two entirely different methods by which the augites 

 are converted into hornblende, the direct paramorphic transition and 

 the indirect uralitization ; but why the change sometimes takes place 

 in the former way and at other times in the latter, I have as yet 

 found no clue for determining. 



When a schillerized mineral undergoes alteration by weathering, 

 the characteristic secondary enclosures sometimes remain unchanged. 

 In the case of the diallages of some of the Mull Gabbros, so much 

 titanoferrite is present in their secondary enclosures that the whole 

 acquires, by weathering, the peculiar white and opaque appearance 

 of leucoxene or titan omorphite (see Plate VII. fig. 6). 



The products of the change of enstatite are, in most cases, 

 quite different from those of augite, and more nearly resemble 

 the materials produced by the alteration of olivine. This probably 

 * Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 254, &c. 



