74 DISCOVERY REPORTS 



The ten remaining stones of the acid volcanic series consist of coarse quartz-feldspar-porphyries 

 and their tuffs or igneous breccias, in which a progressive series of cataclastic deformations have taken 

 place, resulting in the formation of typical ' porphyroids ' and, finally, a completely mylonized rock 

 which can only be distinguished with difficulty from a metamorphic quartzite. While the majority 

 of the porphyroids and igneous breccias consist of quartz-feldspar-porphyry fragments only, three 

 contain fragments of rhyolite, felsite, and oligoclase-andesite in subordinate amount. 



The series begins with an almost normal, practically unstressed quartz-feldspar-porphyry or 

 granite-porphyry, containing very abundant phenocrysts of quartz, some a centimetre in length, 

 orthoclase not quite so large, and still smaller crystals of albite-oligoclase, in a fine-grained ground- 

 mass of aplitic type which consists of equidimensional crystals of quartz, orthoclase, and albite- 

 oligoclase. A few small crystals of altered biotite and a little iron ore represent the only ferromagnesian 

 constituents. The phenocrysts collectively make up considerably more than half the volume of the 

 rock. Only the large quartz crystals show the beginnings of stress. They are cracked and somewhat 

 rounded, with narrow zones of granulation along the fissures. 



Next comes a series of rocks which may be described as igneous breccias consisting of shattered 

 fragments of the above quartz-feldspar-porphyry with, in some cases, a few pieces of rhyolite, quartz- 

 felsite, and oligoclase-andesite. These may, perhaps, be best interpreted as explosion breccias, but 

 they may possibly represent scree material at least in part. All these rocks have been subjected to 

 crushing and shearing stress of varying degrees of severity. The quartz phenocrysts have been 

 shattered and ground-mass material has been forced in between the fragments. Sometimes the 

 fragments have not been so far separated that the outline of the original phenocryst cannot be traced, 

 but in more severe cataclasis the fragments have been dispersed far and wide throughout the ground- 

 mass. Where the stress has not been great the feldspars have retained their crystal forms, but have been 

 more or less completely sericitized. With more severe shearing the feldspars have been broken down 

 and may show more or less rounded fragments enclosed in areas of comminuted and sericitized 

 material. In extreme cases the feldspars are represented merely by elongated areas of sericitized 

 material the margins of which fade out gradually into the ground-mass. The ground-mass itself has 

 been sheared and sericitized in the same way, but owing to its finer grain and its consequent greater 

 mobility under shearing stress, it has been forced to flow round the phenocrysts, producing what 

 Quensel {op. cit. supra) has called secondary flow structure. The rocks are then typical 'porphyroids', 

 with elongated strips of felted sericite flakes winding round the broken phenocrysts. Secondary 

 epidote and chlorite have been produced in some quantity, especially in the breccias that contain 



andesite fragments. 



What appears to represent the final stage of cataclastic deformation is reached in a quartzite-like 

 rock which, if seen in isolation away from the associated types, would certainly be regarded as a 

 metamorphic quartzite or quartz-schist. It consists of alternating strips of coarse and fine quartz 

 crystals. Some water-clear plagioclase feldspar is mingled with the quartz of the coarse layers, and a 

 very pale green, almost isotropic chlorite with the fine-grained quartz. The larger quartz crystals 

 interlock with their neighbours along crenulated margins. Chlorite and ilmenite decomposing to 

 leucoxene are somewhat concentrated in restricted areas presumably where fragments of andesite 

 occurred in the original breccia. Patches and veins of clear recrystallized calcite also occur. Not a 

 trace of sericitization is left. Presumably the sericite, together with particles of iron oxide, has been 

 reconstituted into chlorite. This rock is somewhat tentatively identified as the mylonized end-product 

 of extreme cataclastic deformation aflfecting a breccia composed of acid igneous rocks. 



The connected series of rocks above described is thus regarded as a complex of acid lavas, or lavas 

 and intrusions (quartz-feldspar-porphyry, quartz-felsite, rhyolite, dellenite, dacite, and oligoclase- 



