_26o THE CONSTITUTION Ol" CERTAIN KOCKS OF THE AKCILEAN AGE. 



-comes next, with the hornblende in much less quantity. The 

 felspar is usually present in huge crystals, and so is the quartz, 

 .but the latter mineral does not quite attain in size that attained by 

 ,the felspar. Ihese larger constituents are set in a microcrystalline 

 iidmixture of felspar, quartz and hornblende. The phcnocrysts of 

 jelspar and quartz appear to be pulled out longitudinallv and are 

 at times found to be corroded. The hornblende, which is usually 

 SI dark greenish black variety, occurs usually in irregular patches, 

 tcldom of any size, and less frequently in laths which are, when 

 found, of small dimensions. Its similarity in mineralogical com- 

 j)osition to that of the other rocks already mentioned cannot be 

 .doubted. At times there is to be seen a flake of broAvn biotite. 

 'I'he quartz does not contain so many inclusions as in many of the 

 other rocks instanced. The phenocrysts of felspar rarely show 

 .any types of twinning, but where this does occur it takes place 

 either after the Albite Law or after both the Albite and Carlsbad 

 Laws. Among the microcrystalline admixture of felspar, quartz 

 and hornblende that fills up the interstices between the phenocrysts 

 of felspar and quartz twinning in the felspars, although of more 

 frequent occurrence than in the phenocrysts themselves, is scarcely 

 -noticeable, but where it does occur it is usually after the Albite 

 Law and seldom after the Pericline Law. The quartz crystals or 

 granules bear evidence of having endured some strain. This 

 type of rock must have been originally of a porphyritic nature, and 

 it must have suffered from metamorphism. Well-defined cleavage 

 planes are noticeable in many of the felspar phenocrysts, and 

 some of the specimens are very pure, but others show traces of 

 decomposition taking place. As in the case of the other rocks 

 already mentioned when decomposition starts it takes its rise 

 first in the cleavage cracks, and with similar results. The fels- 

 pars, as with the quartz granules, are often traversed by irregular 

 cracks, and this is more noticeable in the large phenocrysts. With 

 reg"ard to the arrangement of the minerals in the micro- 

 crystalline base, it may be worthy of notice that the coarse- 

 grained particles are always confined to the central part of the 

 interstices and the finer-grained particles lie between them and 

 the phenocrysts. It often happens that the quartz and felspar 

 have crystallised together within certain areas in those parts where 

 the base is most finely microcrystalline, and have thus given rise 

 to a micropegmatitic structure. These patches are always ad- 

 jacent to the sides of the quartz or felspar phenocrysts. It mav 

 be a case of the vitrification and recrystallisntion of the small 

 particles through the metamorphic influences. The felspar, which 

 is triclinic, seems to range with regard to basicity in the neigh- 

 bourhood of that of labradorite. Let us now look at the micros- 

 copical structure of the black coloured rock, which is associated 

 Avith the above tvpe of rock. It docs not differ as regards its 

 mineralogical constituents from those above mentioned, saving 

 that, although holocrystalline in character, it consists for tlie most 

 part of the microcrystalline base of the above type. It is also 

 found to contain phenocrysts of felspar and quartz scattered 

 throughout its mass, but thev are comparatively few in number 

 and small in size compared to those in the other tvpe of rock, 



