372 THE VERMILION IRON-BEARING DISTRICT. 



or post-Keweenawan age, and will be described under the heading 

 "Keweenawan" in Chapter VI. On the other hand there are dike rocks 

 which cut all the rocks of Archean and Lower Huronian age, but no 

 definite proof has been obtained that they intrude any of the rocks younger 

 than these. The much greater age of these dikes is shown in their more 

 extensive alteration, indicated macroscopically by their green color, and by 

 the occasional presence of an imperfectly developed schistosity. The orig- 

 inal characters of these dike rocks are rarely well enough preserved to enable 

 one to determine positively just the kind of rocks they are. It can be said 

 in general that they are of basic and intermediate character. Some have 

 unquestionably been derived from dolerites and basalts. Others, it is clear, 

 belong to the lamprophyric rocks. 



The dolerites and basalts are invariably very much altered. Occasion- 

 ally a fairly well-preserved ophitic texture may be observed. Usually, 

 however, all textures have been destroyed as the result of orogenic 

 movements, and the rocks have become fairly schistose. They were 

 evidently intruded, however, in the Archean rocks after the latter had been 

 subjected to pressure, as they are found in some cases to have been injected 

 parallel to the schistosity of these rocks. The usual constituents are such 

 as are commonly found in these altered basalts: actinolite, chlorite, apatite, 

 calcite, muscovite, feldspar, a little quartz, sphene, and iron oxides. 



The lamprophyric dikes above referred to occur usually in very narrow 

 dikes and while ordinarily extremely altered, nevertheless are generally not 

 so much altered as are the dolerites. These rocks consist of various 

 combinations of plagioclase and orthoclase feldspar, Avith biotite, hornblende, 

 augite, and iron oxide. The hornblende and augite are the predominant 

 dark silicates. A few serpentinous areas indicate the former presence of 

 olivine. The minerals are so much altered that a trustworthy separation 

 of these rocks into the various divisions of the lamprophyres to which they 

 belong could not be made. There seem to be represented among them, 

 however, chiefly biotite-kersantites, augite-kersantites, and the hornblende- 

 and augite-vogesites. With these there also seem to be some camptonites. 



Certain other dikes in the district which were observed were so 

 extremely altered that one could only see that the original rock carried 

 hornblende, but the petrographic position of these dikes could not be 

 determined. 



