158 



we attempt to determine the precise nature of the white spots. They are often 

 almost opaque in thin section, and never show any definite form or cleavages. 

 In some cases they exhibit a certain amount of transparency, and are then seen 

 to be brown in colour and to possess moderately strong double refraction. 

 Professor PUMPELLY describes the occurrence of similar spots in the felspars 

 of a diabase from Eagle river, and refers them to prehnite. (1) Whatever the 

 mineral forming these spots may be, it is a constant or nearly constant feature 

 in the Lizard saussurite, which thus does not answer to the description of 

 saussurite as given by CATIIREIX. The other recognisable constituents in the 

 Lizard saussurite are felspar, epidote, tremolite and actinolite. There is also 

 very often a minutely granular substance giving aggregate polarisation. 

 Different specimens of saussurite differ considerably in the relative proportions 

 of the different constituents. Indeed it must be remembered that the term 

 saussurite has no precise signification. It is merely employed to designate 

 the dense light-coloured aggregates which arise in connection with the 

 alteration of a basic felspar. Mr. HUDDLESTON gives the following analysis 

 of a mass of saussurite from a coarse grained gabbro which occurs near 

 Caerleon Cove. (2) 



Si0 2 ... ... ... ... ... 45-70 



Al,0 3 ... ... ... ... ... 23-00 



Fe 2 3 ... ... ... ... -50 



CaO ... ... ... ... ... 19-30 



MgO ... ... ... ... 4-75 



Alkalies, &c., (diff.) ... ... ... 1-95 



Ig. ... 4-80 



100-00 



He remarks " that the compound has more affinity, as regards its percentage 

 of silica, with anorthite than with labradorite ; but the presence of so much 

 magnesia, and its poverty in alkali must lead us to regard it as a mixture 

 rather then as a mineral species worthy to be classed with the felspars." The 

 magnesia in the above analysis is accounted for by a pale coloured hornblende 

 which was recognised by Professor BONNEY in the material submitted to 

 Mr. HUDDLESTOX for analysis. If we compare the analysis of Lizard 

 saussurite with the analyses of various saussurites by DAMOUR, HUNT, 

 BOULANGER and others (S) we are struck by the fact that it contains less silica 

 and alkali, and more water than the average. The original felspar was, as 

 Mr. HUDDLESTON suggests, probably more allied to anorthite than to 

 labradorite. The presence of over 4 per cent, of water accords with the 

 view that the doubtful mineral which was referred to in describing the 

 microscopic structure of the substance is a lime-zeolite. 



Associated with the exposure of actinolitic gabbro in Sangomore Bay, 



(1) Proc. Amer. Acad. 1878, p. 272. 



(2) Q.J.G.S., vol. XXXIII., 1877, p. 927. 



(3) See RAMMELSBEEO. Handbuch der Mineralchemie, p. 566. 



