or Saussurite-smaragdite Gabbro of the Saasthal. 241 



coarser varieties of the former appeared either in streaks or in 

 vein-like masses in the ordinary rock, without, however, any 

 sharp line of demarcation. In fact (as I wrote in my note- 

 book at the time) all the varieties of gabbro seen on the ridge 

 " seem to change rapidly one into another without any 

 marked divisional lines, just as the coarser and finer, or the 

 more and the less felspathic masses of the Lizard gabbro pass 1 

 one into another — that is, they are all variations of one mass," 

 the distinctive characteristics being due to differences, pro- 

 bably slight, either of chemical composition or of environment. 

 The small outcrop on the southern side of the snow-saddle 

 consists mainly of the hornblendic variety (4), and that on 

 the northern side wholly, so far as we saw. A specimen 

 brought by Mr. Eccles from the exposure in the spur of the 

 Allaleinhorn shows this to be the same rock. 



So much has been already written upon the microscopic 

 structures of gabbros, and the varieties of rock into which 

 they pass, that a brief notice will suffice for most of those from 

 the Saasthal. In the specimens which I have examined the 

 " saussurite " (as is usually the case) appears to be a rather 

 variable aggregate of minute minerals. Among these, two 

 kinds can be often distinguished : one varying from fibrous 

 to prismatic, fairly clear, giving, with crossed nicols, bright 

 tints with a fibrous-speckled aspect ; the other, rather more 

 distinctly fibrous and occurring in patches of a dusty aspect, 

 but feebly translucent, and apparently having weak double- 

 refractive power, for the tints are duller. In one or two 

 specimens, however, notably in the variety (4), the con- 

 stituents are larger and better defined. Here zoisite may be 

 recognized abundantly in prisms sometimes about "03 inch long, 

 with a fairly marked pinacoidal cleavage, and an occasional 

 basal one. It is very slightly fibrous in structure, and gives 

 low polarization-tints with straight extinction. There is also 

 some colourless epidote, a flake or two of white mica, and a 

 little interstitial quartz. Here and there small patches of 

 fibrous hornblende are interspersed. So this " saussurite " 

 consists of more than one microlithic mineral, and is un- 

 doubtedly an alteration-product of a lime-felspar*. 



The smaragdite in the hand-specimens often shows distinct 

 cleavage-planes, sometimes resembling those of a diallage, as 

 if it occurred in large crystals ; but we find these on micro- 



* " If, then, we combine the researches of Lossen, Kloos, and Cathrein, 

 we seem justified in concluding that a basic felspar may be replaced by 

 albite and epidote, zoisite, or a zeolite of the scolecite group." Tealf, 

 1 British Petrography,' p. 152 (et ante). 



