296 J. A. THOMSON 



Two rocks in the Helms' collection are possibly to be 

 referred here. The first is labelled " 12 miles N.W. of Camp 

 23, 17/7/91," i.e., in the northern part of the Blyth Range. 

 It has the appearance and mineralogical composition of a 

 hornblende granite, bat some peculiarities in structure, 

 though neither hand specimen or section show any parallel 

 structure. Both orthoclase and quartz are abundant, 

 together comprising the bulk of the rock, and the first 

 peculiarity is the relation of these two minerals. Large 

 pseudoporphyritic plates of orthoclase are found enclosing 

 small rounded grains of quartz in poecilitic fashion (Fig. 2, 

 Plate XIV). Such a structure, if original, as there seems 

 no reason to doubt it is, may be explained by the fact that 

 the magma originally contained quartz in excess of that 

 required for the quartz-felspar eutectic. 1 The structure 

 is, however, further complicated by the presence of a thin 

 zone of quartz-felspar intergrowth between the host and 

 tlie enclosed mineral. Outside the large plates of ortho- 

 clase such intergrowths are very abundant, but are always 

 of fine grain, and have a great tendency to resemble grid- 

 irons rather than the script-like forms that have given rise 

 to the term 'graphic.' Their presence between the ortho- 

 clase and enclosed quartz suggests that they are not 

 original, but of the nature of ' myrmekite,' a type of struc- 

 ture which in Sweden and Finland is taken to prove great 

 metamorphism and the Archaean age of the granite. 2 Besides 

 orthoclase there is also a smaller amount of microcline and 

 oligoclase, both bounded by similar intergrowths. The 

 oligoclase is sometimes included within the orthoclase. 

 All these minerals occasionally show strain shadows. The 



1 Of., quartz in oligoclase. Dwerryhouse, A. R., On some Intrusive 

 Bocks in the Neighbourhood of Eskdale (Cumberland), Q.J.G-.S., lxv 

 1909, pp. 63 and 70. 



2 Holmquist, J. P , Studien iiber die Granite von Schweden. Bull. 

 Geol. Inst, Upsala, vn, 1904-5, Nos. 13, 14, p. 116. 



