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170 



The only constituent which remains to be considered is the interstitial 

 matter. In the rocks of the gabbro-f amily this is entirely absent. In the dolerites 

 and basalts it may or may not be present. The composition of the interstitial 

 matter in any given case depends upon the composition of the original 

 magma, and the character and amount of crystal-development which has 

 taken place during the process of cooling. In some cases it is a rich brown 

 homogeneous glass ; more commonly it is a more or less colourless glass 

 crowded with crystallites, microlites or skeleton crystals. By a gradual 

 increase in the amount of glass the crystalline pass into the vitreous rocks. 

 In the holocrystalline plagioclase-augite rocks with ophitic texture we have 

 evidence that progressive crystallisation gave rise to the formation of 

 interstitial matter which finally crystallised as augite. As a rule a 

 homogeneous brown glass is rare in basic igneous rocks. Crystallites and 

 microlites indicate that a certain amount of differentiation took place after 

 the formation of definite crystals had ceased. 



DESCRIPTION OF THE ROCKS. 



In dealing with this portion of the subject we shall adopt that method 

 of treatment which appears to accord best with what is known as to the 

 geological relations of the different rock masses. The absence of homogeneity 

 in rock masses which possess a certain amount of individuality so far as 

 their mode of occurrence is concerned renders a rigid system of classification, 

 based on mineralogical composition, unsuitable to the expression of geological 

 facts. As matters stand at present the basic rocks which contain a 

 considerable amount of original hornblende may be conveniently separated 

 from those which consist essentially of plagioclase, olivine and pyroxene, or of 

 secondary products (including hornblende), derived from these minerals. 

 The former group, so far as bulk is concerned, appears to be completely 

 subordinate to the latter. If we fix our attention on the holocrystalline 

 representatives of the latter group we recognise the existence of two extreme 

 types the plagioclose-olivine rocks and the plagioclase-pyroxene rocks 

 and also of a number of intermediate forms, composed of the three essential 

 minerals in varying proportions. If we wish to indicate the quantitative 

 relations of the minerals in these intermediate forms we may do so by placing 

 the names in the order of the relative importance of the minerals which they 

 represent. Again, further complication is introduced by the fact that the 

 pyroxenes may be monoclinic or rhombic. The former are usually designated 

 by the general term, augite ; the latter by the term, enstatite. The two 

 pyroxenes may occur singly or together. If now we consider the textural 

 characteristics of the basic eruptive rocks we notice every gradation from the 

 coarsely c^stalline variety, in which the sizes of the individual have to be 

 measured by inches, to the compact variety in which the crystalline 

 character of the rock can only be ascertained by the use of the microscope ; 

 and also every gradation from the holocrystalline to the vitreous condition. 

 Moreover, every kind of rock has its porphyritic and non-porphyritic 

 representatives. Having regard to the transitions which occur in every 



