120 DE. J. W. EVANS ON THE EOCKS OF THE [Feb. I906, 



conditions. Occasionally these later crystals appear to have pushed 

 aside the granulitic element, as is sometimes the casein the rocks of 

 Morrinhos and Santo Antonio, but more usually they have, in the 

 course of their formation, enveloped the pre-existent granules which 

 are now found embedded in them. In some cases, the granulitic 

 phase appears to have been of comparatively-short duration ; 

 in others, the greater part, if not the whole, of the rock must 

 have separated out before the conditions allowed of the develop- 

 ment of the granitic structure. Sometimes the granulitic forms 

 pass, by intermediate gradations, into those of the normal idiomorphic 

 or hypidiomorphic granitic type ; sometimes there is an abrupt 

 change from one to the other. 



In ordinary granitic rocks the quartz is the last mineral to 

 crystallize, and has no crystalline boundaries of its own, being 

 entirely allotriomorphic ; but, throughout the granulitic element in 

 the rocks which I am describing, a large proportion of the quartz 

 shows idiomorphic boundaries, rounded it is true, but still recog- 

 nizable. This is most clearly seen where the quartz-granules occur 

 as inclusions in felspar, but it can be recognized even where the 

 whole rock is made up of granulitic crystals. 



Idiomorphic quartz is also found in many volcanic and dyke- 

 rocks where the quartz appears to have crystallized out at least 

 as early as the felspars. The most probable explanation of the 

 difference in the order of crystallization in these and deep-seated 

 granulitic rocks, is that it depends on the pressure to which the 

 magma is subjected. 1 We may, therefore, reasonably suppose that 

 the granulitic rocks of the cataracts crystallized at first under com- 

 paratively low pressure, and were subjected to long-continued earth- 

 movements, which ground together and rounded the crystals during 

 the course of their formation, or in the period immediately succeeding 

 it ; although the superior hardness of the quartz enabled it to retain, 

 in some cases at least, traces of its crystal-outline. The finer debris 

 resulting from the process of rounding must have been reabsorbed 

 by the magma. Indeed, in some cases, as in the rock of the 

 Esperanza Cataract, we have evidence of the quartz-crystals having 

 been attacked and partly dissolved. This resorption may be ex- 

 plained by the increase of temperature which might be expected 

 under the conditions that prevailed. For the prolonged earth- 

 movements would themselves generate a considerable amount of 

 heat, and the foldings in which they resulted would cause the 

 accumulation of mountain-masses and a consequent rise in the 

 isotherms. The cooling and crystallization of the still imperfectly- 

 consolidated rock would then recommence, and proceed more slowly 

 and under greater pressure than before, as well as under less- 



1 J. A. Cunningham, ' A Contribution to the Theory of the Order of 

 Crystallization of Minerals in Igneous Rocks' Sci. Proc. Roy. Dublin Soc. vol. ix 

 (1900-1902) p. 383. The presence of water may also have considerable 

 influence, as has been pointed out by Prof. W. J. Sollas, Geol. Mag. 1900, 

 pp. 295 et seqq. 



