266 ME. w. &. siiAXXOX o>' A [vol. Ixxviiiy 



serpentine ; it is nearer actinolite tlian tremolite. The specific 

 gravity =2 -So. 



(c) Boston ite-veins. — In hand- specimen these are bluish 

 grey in the fresh rock and felspathic in appearance. In thin 

 section the rhombs of plagioclase are conspicuous. This is of two 

 kinds : one with albite-lamell^e well developed, and the other with 

 the peculiar streaky appearance of soda-orthoclase. All gradations 

 can be followed, from coarse checjuer-structure to the finest micro- 

 cline ; often there is a central chequer-crystal siuTOunded by 

 chlorite, in turn surrounded by a newer growth of soda-orthoclase. 

 The chequer- structure indicates an albite ; but a Becke determina- 

 tion of the refmctive index shows that this is higher than in 

 albite, being slightly above that of the balsam of the slide. 



Extinctions are not easily measured, but appear to be nearly 

 straight : probably the felspar is referable to potash-soda-oligo- 

 clase-albite. A fair amount of orthoclase occurs in anhedral 

 grains. 



Chlorite occurs between the crystals, as well as inclusions. 

 Prehnite is an alteration-product of the felspar ; there are also a 

 few small grains of epidote, and tremolite in outer portions. 



The rock approaches nearly to the salic variety of the albite- 

 dolerite of Trusham as regards the felspars ; it approaches the 

 bostonites from the point of view of the scarcity of the ferro- 

 magnesian minerals. The following Eosiwal analysis is compared 

 with these rocks : — 



I. II. III. 



SiOs o3-59 



AloOg 17-80 



FeO & Fe--.03 . 11-82 



MsO 2-U 



Cat) 1-34 



fS) ■■::::::::: } ^-^^ 



52-00 



58-47 



13-06 



16-11 



7-32 



7-75 



2-84 



1-58 



4-59 



0-94 



4-68 



5-18 



3-78 



4-34 



Totals 96-81 93-27 9437 



Specific gravity = 2-66. 

 I. Bostonite-veiu, Knowles Hill. Eosiwal analysis. 

 IL Bostonite, Onston Xess. Orkneys. ^ 

 III. Albite-diabase, felspathic variety, Trusham.- E. G. Radley. 



In I, 46 per cent, albite was calculated as Ab^An^ 22 per cent.,, 

 soda-orthoclase as 17 per cent. Xa-f K, and T'o per cent, orthoclase, 

 the felspar totalling 76*6 per cent, of the rock. An analysis was 

 conducted on the same rock by means of a Harada-Brogger tube 

 and the felspar separated by the Sonstadt solution ; the felspar and 

 the heavy minerals were weighed, about 2 grammes of jDOwder 



^ Quoted by Dr. F. H. Hatch, ' The Petrologv of the Igneoiis Eocks '" 1914, 

 p. 240. 



^ •' Geology of the Conntry aroimd Xevrton Abbot' Mem. Geol. Siirv, 1913, 

 p. 62. (Both analyses are modified.) 



