ANDESITES OF DEYONSHrRE. 503 



dark reddish to bluish purple, frequently resembling that of some 

 specimens of Cambrian slate from Penrhyn Quarry, North Wales. 

 Texture almost compact, with here and there a porphyritic plagio- 

 clase (Nos. 912 a, 913, 914, 915, 928 a & b, 929 a & c). Quartz 

 inclusions are abundant in some specimens, as in Nos. 912 a, 913, 

 928 A & B, and present in Nos. 914, 915, 929 a & c. In the case of 

 the four rocks just nientioned, ferruginous pseudomorphs after olivine 

 are visible. 



Type 2. Light reddish or pinkish purple to brown weathered 

 rocks, full of vesicles drawn out in the direction of flow. Speci- 

 men No. 889 is as vesicular as a sponge, and is crowded with small 

 red or yeUow pseudomorphs after olivine. Specimens No: 897 (from 

 upper surface of bed) and No. 908 b do not contain such large 

 vesicles as No. 889, and few pseudomorphs are. visible in them. 



Type 3. This includes the great majority of the specimens, which 

 are evidently weathered rocks varying from dark purplish red to 

 lilac or pinkish in colour, sometimes very amygdaloidal, as in 

 No. 912 B (within 3 feet of the upper surface of the bed) and 

 No. 904, but often with few or no amygdules, E,eddish pseudo- 

 morphs after olivine are often visible, e.g. in Nos. 899 a & b, 916, 

 918 A, B, & D, 929 B. 



As to the microscopical character of these rocks, their structure is 

 porphyritic in the sense of Eosenbusch.^ 



Phenocrysts. — Olivine and in some cases plagioclase. Brown mica 

 in No. 889. 



Groundmass. — Plagioclase. Augite in Nos. 914, 915. Magnetite 

 or ilmenite. Brown mica in No. 889. 



(i) Olivine Phenocrysts. — Unaltered olivine, colourless in thin 

 section, does not occur. The olivine has in most cases undergone the 

 alteration described by itosenbusch,^ it is " changed to a red trans- 

 parent substance of laminated structure and mica-like habit ; the 

 lamellae appear to lie parallel to the brachypinacoidal (010) cleavage- 

 cracks. The substance is very distinctly pleochroic, and absorbs 

 rays vibrating parallel to the lamination much more strongly than 

 those perpendicular to it. The colour of this substance is interme- 

 diate between that of specular iron ore and that of pseudobrookite '^ 

 [translation]. The change just described is very well shown in 

 Nos. 928 a, 914, and 929 a. 



The pseudomorphs are often a dark reddish-brown in colour and 

 semi-opaque. In some cases the margin consists of magnetite, as 

 in Nos. 909, 911, 9 18 a. Sometimes almost the whole of the original 

 crystal is replaced by a granular aggregate of a zeolite showing 

 the grey interference-colour of the first order, as in No. 918 d, and 

 accompanied by calcite in No. 929 a. The pseudomorphs in No. 928 a 

 consist of calcite with a reddish-brown margin of the material before 

 mentioned, described by Eosenbusch. 



Section 929 a illustrates excellently every intermediate stage 

 between forms 1*15 millim. in diameter with reddish-brown trans- 



1 Op. supra cit. p. 340. - Ibid., p. 489. 



