J. H. Teall — Cheviot Quarfz-Fekites and Augite- Granites. 109 



diffei'ent elements of the splierulite may have diflferent positions in 

 relation to the spherulite as a whole. Under crossed Nicols such a 

 structure may give rise to more or fewer arms than the number 

 characteristic of true spherulites, and these arms may not lie parallel 

 to the vibration planes of the Nicols. Such structures, wherever 

 they may occur, Professor Eosenbusch proposes to call pseudo- 

 spherulites. 



The.granophyres, then, are distinguished by the presence of a 

 micro-pegmatitic, pseudo-spherulitic or some allied structure in the 

 ground-mass ; whereas the ground-mass of the micro-granites is an 

 irregular aggregate of crystalline grains. Rosenbusch also uses the 

 adjective granophyric to express the various modifications of structure 

 which occur in the ground-mass of the granophyres. It is some- 

 what unfortunate that the terni granophyre should have been used 

 in two senses, but no confusion will arise if we remember that the 

 granophyre of Vogelsang comprises the micro-granite as well as the 

 granophyre of Rosenbusch. It is always advisable in describing 

 rocks and rock-structures to refer to typical examples. The rock 

 from the summit of Carrock Fell, Cumberland, which was well 

 described by Ward ' as a spherulitic felsite, is a most beautiful 

 granophyre, as that term is employed by Rosenbusch. The ground- 

 mass sometimes shows the micro-pegmatitic structure only, and 

 sometimes the pseudo-spherulitic ; the two structures often co-exist 

 and together they produce the granophyric structure of Rosenbusch. 

 This rock is also interesting on account of the presence of well- 

 formed crystals of a monocliuic pyroxene. It is identical with the 

 granophyre ^ of the Fontaine Laquainte, Kirneckthal, Dep. des Vosges, 

 which occurs as a dyke in the andalusit-hornfels of that district. 



Between the micro -granites and granophyres on the one hand, and 

 the vitrophyres on the other, we have a group of rocks (the felso- 

 phyres of Vogelsang) about which the most diverse opinions have 

 been held. It is with reference to these that the terms micro-felsite 

 and crypto-crystalline have been used. They can be defined only 

 by employing negative characters. Their ground-mass is neither a 

 true glass nor a recognizable aggregate of crystalline grains. When 

 examined under polarized light, it may or may not appear perfectly 

 isotropic. If isotropic, it is distinguished from a glass by the fact 

 that it appears to be composed of ill-defined flecks, scales, fibres, and 

 granules. To this Rosenbusch proposes to restrict the term micro- 

 felsite, and there is no doubt that if this proposition be generally 

 accepted, it will tend to facilitate verj' greatly the precise description 

 of rocks of this class. If not isotropic, then it is distinguished from 

 the micro-crystalline ground-mass by the apparent absence of definite 

 boundaries to the doubly refracting particles. The term crypto- 

 crystalline is applied to this structure. 



Now in dealing with this cr3'pto-crystalline ground-mass, we ex- 

 perience precisely the same difficulties as those experienced by the 

 older petrographers in dealing with the felsitic base. The introduc- 

 tion of the microscope has not removed the difficulties ; it has only 

 ^ Q,. J. G. S. Yol. xxxii. p. 20. ' Die Steiger Schiefer, p. 345. 



