ARCHEAN GRANITES. 253 



Sericite-schists. — When tlie crushing and accompanying' alteration are 

 considerably advanced, sericite-schists are produced from these porphyries 

 and granites. At one stage we find a few eyes of quartz and feldspar left. 

 These lie in a finely granular groundmass in which the parallel structure of 

 the secondary minerals is very e^ddent. This parallelism is most pro- 

 nounced Avheu the rock contains a great deal of sericite, for then the plates 

 of sericite are arranged parallel to one another and greatl)^ emphasize the 

 structure. The parallel arrangement follows around the phenocrysts, 

 showing that it was produced after their formation. It may be that in 

 some cases this parallelism represents partly an original flow structure which 

 has been emphasized by the production of the secondary minerals. The 

 extreme stage shows a very fine-grained schistose rock which is of a 

 yellowish-green color macroscopically, and which under the microscope is 

 seen to be a finely granular aggregate of quartz, presumably some feldspar, 

 and flakes of sericite, these being the predominant minerals. 



Chlorite-scMsts. — In a number of cases the porphyries show abnormal 

 alteration to a green chlorite-schist instead of to a sericite-schist. Such 

 alteration was found in immediate association with the iron-bearing forma- 

 tion — that is, where the acid rocks had been intruded and then infolded in 

 the iron formation. The production of the chlorite and of the green color in 

 general is due to the infiltration of iron from adjacent formations. In some 

 instances the green coloration is found only along the contact of an acid 

 dike with the iron formation and extends only a few inches into the 

 porphyry. In other cases, where the acid rock is in the midst of the 

 iron formation, the rock is distinctly green, and might be, and in fact has 

 been taken for a product derived from the altered basic rocks of the area — 

 the greenstones. In some places the quartz phenocrysts have been granu- 

 lated, but in others they are still intact and show clearly the fact that 

 these schists were derived from acid rocks. The effect of the iron in 

 causing the production of chlorite instead of sericite can be seen in many 

 places in the massive acid rocks. In these we ver)^ commonly find that 

 when iron pyrites occurs it is almost invariably surrounded by a zone of 

 limonite of variable thickness, and beyond this zone of limonite there is 

 a chloritic zone in the groundmass, whereas elsewhere seiicite occurs and 

 not chlorite. 



Schistose granites and schists derived from granites. — Locally these gran- 

 ites have been very much crushed, and as a result of this crushing there 



