386 SIR A. GEIKIE ON THE TERTIARY [May 1896,- 



progress of his mapping in the Strath district of Skye, had occasion 

 to go over a number of the localities (Creaghan Dubha, etc.) cited by 

 me, and, while corroborating my general conclusions regarding them, 

 has been able to obtain much fresh evidence regarding the nature 

 and extent of the metamorphism which the bedded basalts have 

 undergone. His results will appear in due time, when the survey 

 of Skye is further advanced. I have submitted to him some slices 

 cut from typical examples of the altered plateau-basalts as they 

 approach the gabbro of Loch Scavaig, and he has supplied me with 

 the subjoined report regarding them : — 



' In hand-specimens the bedded basalts from the neighbourhood 

 of the gabbro of Loch Scavaig [6613-6618] do not appear very 

 different from the normal basalts of this region . The most conspi- 

 cuous secondary mineral is yellowish-green epidote in patches, and 

 especially in the amygdules. 



' The texture of the rocks varies, and the slices show that the 

 microstructure also varies, the augite occurring sometimes in small 

 ophitic plates, sometimes in small rounded granules. The chief 

 secondary change in the body of the rock is shown by the augite, 

 which is seen in various stages of conversion to greenish fibrous 

 hornblende. Some round patches seem also to consist mainly of the 

 latter mineral, and are probably pseudomorphs after olivine. Here 

 the little fibres are confusedly matted together, without the paral- 

 lelism proper to uralite derived from augite. No fresh olivine has 

 been observed. The felspar and magnetite of the basalts show 

 little or no sign of metamorphic processes, unless a rather unusual 

 degree of clearness in the felspar- crystals is to be regarded in that 

 light. 



' The contents of the metamorphosed amygdules are not always 

 the same. Epidote is usually present in some abundance, and in 

 well-shaped crystals. It has a pale citron tint in the slices, with 

 marked pleochroism ; but a given crystal is not always uniform in 

 its optical characters. Frequently the interior is pale, and has a 

 quite low birefringence: this is probably to be regarded as an 

 intergrowth of zoisite in the epidote, and there are a few distinct 

 crystals of zoisite seen in some places. 



* In the slide which best exhibits these features [6613J the crystals 

 of epidote are in part enwrapped and enclosed by what are doubt- 

 less zeolitic minerals. At least two of these are to be distinguished. 

 One, very nearly isotropic, and with a pale-brownish tint, is pro- 

 bably analcime. Associated with this is a colourless mineral with 

 partial radiate arrangement and with twin-lamellation ; the bi- 

 refringence is somewhat higher than that of quartz, and the 



and it affects indifferently any part of the basalt-plateaux which may chance to 

 lie next to these masses. The bedded lavas can be traced step by step from 

 their usual unaltered condition in the plateaux to their metamorphosed state 

 next to the eruptive rocks. The nature or degree of the metamorphism has 

 doubtless somewhat varied with the composition and structure of the rocks 

 affected, and with the character and mass of the eruptive material ; but it ia 

 certainly not confined to the older parts of the plateaux nor to any supposed 

 pre-basaltic group of andesites. 



