110 ME. C. E. TILLET OX THE GEAXITE-GXEISSES [vol. Ixxvii^ 



pvroxene-ampliibole conversion. This quartz is very generally 

 accom^Danied by orthoclase, Avliich in the normal tyjDe of aniphi- 

 bolite is either absent or very sparingl}' present. The existence of 

 these quartz- and orthoclase-bearing amphibolites calls therefore 

 for some remark. 



At first sight, a number of explanations can be proffered. Either 

 the quartz-orthoclase amphibolites represent hylnid rocks in which 

 a normal amphibolite has been intimately penetrated by quartz- 

 and-orthoclase solutions of the invading magma ; or these rocks are 

 the metamorphosed equivalents of igneous rocks slightly more acid 

 than the normal doleritic tj^pe ; or lastly, the}' are sediments now 

 highly metamorphosed. Of these explanations, the second is the 

 most probable, and is in agreement both with their general com- 

 position and their mode of occm-rence in the field. 



With regard to explanation (1), it is true that the amphibolites 

 have in many places been penetrated by fine strings of pegraatitic 

 solutions from the granite-gneisses, and that the amphibolites may 

 thus locally be converted into hornblende-gneisses. But it is 

 also to be remarked that the amphibolites with which I am now 

 dealing show this content of orthoclase and quartz throughout their 

 masS; and in positions remote from any pegmatitic veining. For 

 example, in many cases such amphibolites are veined only at their 

 borders, the interior and centre of the band being complete!}' free 

 from any isrneous intrusion, vet the centres of these bands show a 

 remarkable uniformity of composition, the minerals in question 

 being present in such an association with the hornblende and 

 plagioclase as precludes any external source. This is the more 

 convincing when the structure of the rock in question shows the 

 presence of potash-felspar and quartz in an undestroyed blastophitic 

 ground-mass, or other typical residual texture. 



It is to be noted that hybridism has played a part in the con- 

 stitution of the amphibolites, as I have alread}' remarked ; but this 

 process is not capable of accounting for the features of the amphi- 

 bolites now under discussion. 



It is of interest to note in the former connexion, that the 

 felspars of the veins and strings of granite very generally show an 

 advanced state of decomposition, while the felspars of metamorphic 

 origin are characteristicalh^ clear and free from decomposition, 

 although there are exceptions to this condition. This differen- 

 tiation in the susceptibility of weathering of felspars of igneous 

 and metamorphic origin respectively, is of wide application, and 

 finds a parallel in contact-metamorphic areas around plutonic 

 intrusions.^ The reason for this is at first sight not very clear, 

 but a partial explanation may perhaps be found in the fact that a 

 removal of inclusions susceptible to decomposition accompanies the 

 recrystallization of metamorphic felspar. 



It was of importance, in the elucidation of these quartz-orthcclase 

 amphibolites, to determine the chemical composition of a typical 



1 See A. Harker & J. E. Marr, Q. J. G. S. vol. xlrii (1891) p. 296. 



