130 ME. J. MORRISON ON [vol. lxxiv, 



The xenocrysts show characters which accord with the fores-oini? 

 theory of their origin. In the more acid varieties the felspars 

 attain a maximum size, and form a large part of the rock. Corro- 

 sion increases progressively, small well-rounded individuals being 

 the rule in the more basic types. Quartz suffers in a similar 

 manner. The successive stages are best followed under the 

 microscope ; hexagonal sections are at first fairly well defined, but 

 the crystals gradually diminish in size, lose all trace of form, 

 and survive as rounded grains. At a certain stage a corrosion-ring 

 appears, increasing in width with the basicity of the rock, and 

 occurring throughout the mass. When flow-structure is present, 

 biotite and pseudomorphs of ferromagnesian minerals are observed 

 tilted against and fractured by collision with the quartz, point- 

 ing to its early crystallization ; and this, together with the violent 

 corrosion, reaction-rim, and the general basicity of the rock, 

 renders the conception of its crystallization from the surround- 

 ing magma scarcely feasible. The orthoclase-xenocrvsts are 

 often changed marginally into an aggregate of plagioclase and 

 quartz, the former orientated with respect to the orthoclase, a 

 feature also observed in the basic patches of the granite. The 

 change has sometimes reached an advanced stage — deep gulfs 

 being formed, separated by narrow sinuous bands of the un- 

 altered mineral. In the more basic examples the orthoclase is often 

 microscopic, and secondary growth is not uncommon. Some of the 

 xenocrysts are microperthitic in places, usually round the edges 

 but sometimes internally, especially near the patches of plagioclase 

 and quartz, and very rarely throughout. A number of sections 

 were cut across porphyritic felspars from various basic patches of 

 the granite for comparison, and microperthite was found to occur 

 in somewhat similar fashion. 



Of the other constituents of these rocks a variety of plagioclase 

 is the commonest phenocryst, and differs from the foregoing in the 

 possession of perfect contours. The largest crystals in the upper 

 members, apart from the xenocrysts, are oligoclase-felspars of a 

 basic type ; while andesine giving symmetrical extinction of 20° 

 is occasionally present. Complex twinning is the rule, and a 

 marked feature is the beautiful zoning exhibited by all the felspars. 

 Oligoclase occurs even in the more basic rocks, but generally as 

 small well-rounded crystals giving low angles ot extinction, and 

 frequently bordered by a rim of orthoclase in optical continuity 

 with that of the ground-mass. These may possibly be derived from 

 the upper layers, thus partaking of the nature of xenocrysts, their 

 insolubility in a basic magma disguising to some extent their 

 origin. A secondary growth round a corroded core is of frequent 

 occurrence, the outer zone being either corroded, especially in the 

 more basic rocks, or presenting more or less perfect outlines, as in 

 the upper more acid rocks. Orthoclase is usually restricted to the 

 ground-mass, where it is associated with a decreasing amount of 

 quartz. 



Biotite is often fresh, and resembles that of the acid group 



