250 THE VERMILION IRON-BEARING DISTRICT. 



Microscopic characters. — The result of mici'oscopic examination sustains 

 the detei'minations made by macroscopic studies. It shows that there are 

 present in the acid intrusives of Vermihon Lake the following petrographic 

 varieties: Rhyolite-porphyry, feldspathic porphyry, micrograuite, granite, 

 microgranite-porphyr)^, and granite-porphyry. The minerals occurring 

 in all of these are essentiall}' the same. Under the microscope quartz is 

 the most prominent primary constituent, and ranges from minute particles 

 taking part in the construction of the groundmass up to phenocrysts an inch 

 in length. Both orthoclase and plagioclase feldspar occur in the groundmass 

 and as phenocrysts. Polysyntheticallj^ twinned plagioclase predominates 

 among the phenocrysts. These feldspars are much altered and an accurate 

 determination of their characters was not made. Brown mica occurs occa- 

 sionally as jjhenocrysts and is almost always altered to chlorite. A few 

 individuals of common green hornblende were observed, which appear to 

 be primary. Apatite, sphene, zircon, and a little iron oxide were also 

 observed. From these various minerals there have been produced by 

 alteration the following secondary minerals: Calcite, which is distinctly 

 ferriferous, chlorite, epidote, zoisite, sericite, muscovite, and rutile. Pyrite 

 in cubes is also commonly found in some of the altered intrusives. The 

 texture of these rocks is normally granitic, although occasionally a tend- 

 ency to a trachytic texture was observed in some of the porphyries and 

 more commonly a micropegmatitic texture was seen. 



No analyses of these intrusive rocks have been obtained. Indeed no 

 special effort has been made to obtain analyses, for the reason that their 

 field associations and general characters show clearly that the various kinds 

 of rock included under the above head belong geneticallj^ together, and 

 for the further reason that the rocks are without exception considerably 

 altered, so that analyses migiit be misleading rather than helpful. It is 

 highly probable that analyses of fresh rocks, could such be obtained, would 

 show that these acid intrusives range from the granites toward the diorites 

 by increase in soda-lime-feldspar and diminution in quartz and orthoclase, 

 as is indicated in some of the feldspathic porphyries. 



These intrusives have been subjected to dynamic action, for they are 

 very commonly jointed, and in some places even rendered schistose. They 

 have also taken part in the folding, but owing to their general homogeneous 



