﻿Vol. 64.] 



EASIC INTEUSION OF BAETESTEEE. 



509 



and (34), where, as seen in the diagram (p. 504), the dyke passes in 

 below the Old Eed. The rock from (31) resembles that from (61) 

 in showing sharply-defined junctions between doleritic and basaltic 

 material (PI. LII, fig, 5), many small patches of dolerite being 

 enclosed in the basalt. 



The following silica-percentages and specific gravities, some of 

 which have been already referred to in the foregoing account, were 

 determined in the chemical laboratory at University College, 

 Bristol, by Mr. J. H. Sturgess : — 



Fosition ioi 

 quarry-face. 



Character of rock. 



Sp. gr. 



Silica- 



per- 



ceniage. 



6 



9 



10 

 11 

 54 



17 



Dolerite, greenish variety from near the \ 

 southern margin J 



2-65 



2-84 

 2-88 

 2-93 

 2-84 

 2-93 



45-45 



4303 



. 45-60 

 48-26 



Olivine-analcime-dolerite (teschenite) 



Basalt 



Basalt 



Basalt 







The characters of the various rock-types which have now been 

 described may be summarized as follows : — 



Dolerites. — The prevailing type is a heavy olivine-analcime- 

 dolerite or teschenite, formed of olivine generally serpentinized, 

 labradorite, fresh non-pleochroic and non-ophitic augite, iron-ores, 

 analcime, and apatite. But the rocks of the prevalent type may 

 vary as regards the amount of olivine, the relative abundance of 

 the granulitic and well-crystallized types of augite, the nature 

 of the iron-ore, and the presence or absence of analcime. 



Near the southern margin is a lighter, somewhat green, finer- 

 grained type generally free from olivine (which, if present, is repre- 

 sented by pseudomorphs in carbonate), and further containing 

 labradorite, augite sometimes fresh (but as a rule altered into a 

 yellow, probably serpentinous mineral), apatite, and occasionally a 

 little biotite. 



Basalts. — The prevailing type has a fine-grained ground-mass, 

 formed of felspar-needles with augite- and magnetite-grains, and 

 includes phenocrysts of felspar (of a less basic type than in the 

 dolerite), and of augite, the latter being very generally corroded and 

 marked by the separation of magnetite. 



Many of the basalts, however, contain olivine, either fresh or 

 serpentinized. 



Small areas of glass are frequent in the basalt, and in one case 

 relatively-large imperfectly-variolitic areas were met with. Small 

 areas of doleritic material are frequently included in the basalt, 

 the boundaries between the two rocks being in some cases sharply 

 defined, in other cases ill-defined. 



2ir 2 



