PETKOLOGICAL NOTES ON FURTHER ROCK SPECIMENS. 311 



This is more particularly noticeable where biotite and hornblende crystals interlock, 

 that is, where it is more definite that the two minerals were formed by the same general 

 alteration of the augite of the dolerite rocks. Epidote, too, is associated with this brown 

 biotite, but is a rim-growth rather than an inclusion. 



The minerals epidote, zoisite, clino-zoisite are usually found with white mica in the 

 saussurite matrix. The lawsonite, however, is not so restricted as the others ; it occurs 

 (in addition to the above manner) as veins which penetrate the amphibole masses and in 

 localised patches in the neighbourhood of the amphibole. Frequently zoisite is found 

 as irregular scales which thread through the saussurite. Under crossed-nicols it is seen 

 to form an ultra-blue background in which the other alteration minerals and relic felspar 

 occur. 



In Nos. 666 and 1240, there is practically no sphene, and no biotite or chlorite. 

 This shows that when the more complex amphiboles are produced in the higher stages 

 of metamorphism they have absorbed into their composition much of the matter of both 

 sphene and biotite. The disappearance of white mica, epidote and zoisite, and the 

 recrystaUisation of the felspar in a more acidic form emphasise the increased complexity 

 of the new amphibole molecule ; and this variation in chemical content is reflected in the 

 different appearance of the mineral. 



3. GENESIS OF THE XENOLTTHS. 



This has been discussed by Stillwell on pp. 53-54 of his work. He immediately 

 (pp. 54-5) proceeds adequately to discuss the significance of the meta-xenoliths. In 

 view of this, nothing further will be given here on these matters. 



4. THE AMPHIBOLITES OF THE METAMORPHOSED DYKE SERIES. 

 This account is concerned with the mineral composition and the microscopical 

 characters of the amphibolite rocks Nos. 632, 633 and 635A. 



No. 632 differs from No. 633 under the microscope. Although it shows some 

 tendency to linearity of amphibole crystal arrangement, yet, on the whole, it is 

 advisable to call its texture granoblastic. In No. 633, however, there is a marked 

 schistosity produced by the parallel orientation of its amphibole crystals. This 

 schistosity is accentuated by the elongation of the crystal prisms whose average elongation 

 factor (length : width) is about 3 ; this is exceeded somewhat in regions where smaller 

 crystals are interwoven with felspar ; it is fallen short of in the more massive parts of the 

 rock. Again, in No. 632, there is a fairly even distribution of parts exclusively 

 amphibolic and parts essentially composed of both amphibole and felspar. No. 633, 

 on the other hand, has these mineral groups arranged in parallel alignment, producing 

 a completely different aspect. 



