504 MR. B. HOBSON ON THE BASALTS AND 



lucent margin and calcite centre (evidently pseudomorphs after 

 idiomorphic olivine), and minute patches 0-09 millim. in diameter or 

 less, with a black margin of magnetite, having sometimes the outline 

 of an olivine crystal, but often irregular and occupied by calcite in 

 the centre with irregular bands of magnetite across it. The same 

 transition is visible in No. 916. These minute pseudomorphs are 

 plentiful in Nos. 929 b, 912 a, 912 c, 913, 928 b, 895 b, &c. 



The pseudomorphs after olivine are frequently corroded by the 

 grouudmass, and very irregular fantastic forms thus arise, as in 

 Nos. 928 A and 916, so that almost all trace of idiomorphism may be 

 lost. In nearly every case the pseudomorphs are fairly abundant, 

 though, as their size is not generally large, they hardly make up 

 a notable percentage of the rock. The maximum size observed 

 was 3*97 millim. in diameter in No. 928 a, but the usual size is about 

 0-8 millim. x 0-4 millim. in the case of the more obvious pseudo- 

 morphs, and about 0*09 millim. diameter in the case of the numerous 

 small magnetite pseudomorphs. 



The only section in which olivine pseudomorphs were not made 

 out was No. 906 a, and it is uncertain whether they are not present in 

 that case as minute magnetite pseudomorphs, since they are present 

 in No. 906. 



(ii) Plagiodase plienoGrysts are present in Nos. 912 a, b, &c, 913, 

 914, 915, 928 a & b, 929 a & b. They give in general nearly square 

 sections, but are never abundant. Though they are idiomorphic, 

 they have been both corroded by the groundmass so as to alter their 

 external contours and minutely honeycombed by the interstitial 

 matter. In some cases the whole interior is honeycombed, in others 

 a central patch remains free from attack, but in nearly all examples 

 there is a narrow outer margin free from honeycombing. I take 

 the explanation of this to be that the crystals were formed in the 

 intratelluric period, that owing to the relief of pressure upon the 

 rise of the rock to the surface they were corroded and honey- 

 combed (as relief of pressure acts like increase of temperature), and 

 lastly, as the rock cooled slowly the crystals renewed their growth, 

 though not vigorously enough to obliterate the traces of the cor- 

 rosion of their contours. 



Brown mica occurs sparingly in No. 889. It is corroded and 

 surrounded by haematite grains (probably altered magnetite due to 

 resorption). 



Groundmass. ■T-V\'8ig\OQ\2i,se, is by far the most important constituent 

 of the groundmass. It occurs in forms giving mostly lath-shaped 

 sections. The largest average size of the crystals observed was in 

 No. 904, namely, -43 millim. x *09 millim. 



The only sections (except Nos. 928 a & b) in which augite occurs in 

 the groundmass are Nos. 914 and 915. In both cases it is distinctly 

 ophitic, but on a very microscopic scale, the allotriomorphic crystals 

 having an average diameter of 0-08 millim. and being usually 

 penetrated by only a single felspar. Traces of augite are seen in 

 Nos. 928 A & B. Calcite, most probably replacing augite, occurs in 

 Nos. 896, 918 a, 928 a & b, 929 a & b. It is accompanied by 



