388 O. E. LE ROY — GEOLOGY OF RIGAUD MOUNTAIN, CANADA 



Montana. In this rock the secondar}^ growth of the quartz is much 

 finer and better answers the definition of a coronal zone than does that 

 of the Rigaud porphyry. In Wei(hnan's description of the Utle}'' 

 metarhyolite* the secondary growth seems to be more identical, in form 

 at least, with that of Rigaud. As this structure in the Rigaud porphyry 

 presents no new features which' have not been previously described by 

 different writers, it is unnecessar}'' to elaborate on the origin again. 



The groundmass is composed principally of quartz and turbid feld- 

 s[)ar,with a large number of idiomorphic grains of magnetite and a few 

 of zircon, plagioclase, green hornblende, and brown biotite. the two latter 

 being partially or wholly altered to chlorite, calcite, and limonite. The 

 structure is microgranitic, and the allotriomorphic grains of feldspar and 

 quartz are gcHerally irregularly^ interlocked. Though very fine in grain? 

 with a high power, the feldspar presents the same appearance as do the 

 phenocrj'sts and is i)robabl3; anorthoclase. 



The quartz-syenite por[)hyry undergoes a variation in the southern 

 part of the area, differinir princii)ally in the base, which megascoi)ically 

 is dull, compact, almost black in color, and weathers to a brownish gray. 

 Microscopically, the feldspar and the quartz show the effects of resorp- 

 tion, but there has been no subsequent growth, the borders of the indi- 

 viduals being quite smooth. The groundmass is an extremely fine 

 grained quartz-feldspar mosaic with a flow structure, accentuated by 

 strings of magnetite granules, which coincide in direction with the flow. 

 From this structure it is evident that this type may be regarded as transi- 

 tional between the quartz-syenite porphyr}'^ and a quartz trachyte. 



Q UA R TZ PORPH YR Y 



The quartz porphyry is separated from the above b}^ a broad band of 

 transition rock which shows a gradual increase in the number of quartz 

 phenocrysts, and a corresponding decrease in the case of the feldspar. 

 In hand specimens the quartz-pori)hyry is a light gray felsitic rock 

 weathering to a pale red, and is thickly studded with rounded pheno- 

 crysts of clear vitreous quartz. Under the microscope the same con- 

 stituents are present that were found in the quartz-s3'enite porphyry, 

 with the exce})tion that the quartz is in very large amount, while the 

 feldspar is represented by only a few phenocrysts in each slide. At the 

 contact of the quartz porphyry and the hornblende syenite the former 

 loses its characteristic structure, and occurs as agranular mosaic of quartz 

 and feldspar, the latter being present largel}'' in the form of Carlsbad twins. 

 This contact facies is a little coarser in grain than would be expected. 



*Pre-Cambrian Igneous Rocks of the Fox Kivor Valley, Wisconsin, 1898, p. •2!t. 



