108 J. H. TcaJl — Cheviot Quartz-Fehites and Auglte- Granites. 



remarks on the subject of the ground-mass of the quartz-porphyries ^ 

 in order that the terms employed may be clearly' understood. 



The characters of this ground-mass have been made the subject of 

 elaborate researches by many observers, and the most diverse 

 opinions have been expressed as to their true significance. The 

 whole subject has been discussed by Rosenbusch, with his usual 

 critical ability, in his work " Die massige Gesteine," - and in the 

 following communication I propose to adopt his terms. 



In any attempt to acquire definite ideas as to the nature of this 

 ground-mass, it is advisable to remember that there are in this, as in 

 every branch of petrography, two moi'e or less independent points 

 of view ; the descriptive and the aatiological. We may consider 

 the actual characters of the ground-mass without reference to their 

 origin, or we may attempt to form some notion of the manner in 

 which they have been produced. From the first point of view two 

 modifications of the ground-mass stand out as the antitheses of each 

 other, and present no difficulty in the way of description. On the 

 one hand we have an isotropic glass, and on the other an aggregate 

 which is capable of resolution, either with a hand lens or with the 

 microscope, into a congeries of perfectly definite crystalline grains. 

 The terms vitrophyre and granophyre were proposed by Vogelsang^ 

 to include those quartz-porphyries in which the ground-mass is pre- 

 sent in one or other of these forms. Eosenbusch,* however, has 

 shown that the granophyres of Vogelsang may be subdivided into 

 two groups, to which he proposes to apply the terms micro-granite 

 and granophyre. The rocks of these two groups are distinguished 

 from each other by the mutual relations of the crystalline constituents 

 of the ground-mass. In micro-granites these constituents are related 

 to each other in the same way as the constituents of granite or 

 granulite. They consist of crystalline grains, mostly of felspar and 

 quartz, which are arranged without any regularity. In granophyres, 

 on the other hand, the quartz and felspar are intergrown according 

 to more or less definite laws, so that definite structures, such as 

 micro-pegmatite, may be recognized in these rocks. One special 

 type of structure which is frequently present in these rocks, and is 

 often seen in association with the true micropegmatite, is termed by 

 Rosenbusch pseudo-spherulitic. True spherulites, according to him, 

 are composed of crystalline fibres of one and the same substance, all 

 radiating from a common centre, and having an axis of elasticity 

 pai-allel with the length of the fibre. They are therefore character- 

 ized under the microscope with crossed Nicols by a perfectly definite 

 four-armed cross, the arms of which lie parallel with the vibration 

 planes of the Nicols, and remain stationary as the stage is rotated. 

 The spherulitic structures of the granophyres are, however, com- 

 posed of quartz and felspar, so that the axes of elasticity in the 



1 It must be remembered that the terms quartz -porphyry and quartz-felsite are not 

 synonymous. The former term is the niore comprehensive, as it inchides such rocks 

 as the porphyritic pitchstones (Vitrophyr of Vogelsang). 



- Page 60. ■* Die Krystalliten, Bonn, 1875, p. 160. 



^ Zusammensetzung und Structur granitischer Gesteine, Z.D.G.tr. 1876, p. 369. 



