On Some Dykes Containing Huron ite. 33 



tral portion. In the plagioclase of the matrix the decomposition pro- 

 ducts are frequently grouped together in the central portion, leaving a 

 comparatively clear and iresh periphery. Certain of the crystals of 

 felspar are quite fresh and glassy, having for some reason escaped the 

 alteration to which most have been subjected. 



With the single exception, perhaps, of the plagioclase originally 

 contained in the fine-grained portion of the rock from Pogamasing the 

 decomposition has not been of such extreme character that secondary 

 albite has resulted and in every other instance the clear felspar sub- 

 stance is certainly an unaltered survival of the original individual. The 

 plagioclase of thegroundmass is usually in more or less elongated forms, 

 but occasionally mutual interference has produced at times rounded 

 contours. In composition to judge from the measurements of the 

 angle contained between the maximum extinction of adjacent lamellae 

 the plagioclase appears to be always near the basic end of the felspar 

 series. Some of the angles obtained are high enough for anorthite, the 

 most basic of the felspars, but generally the angles obtained indicated 

 labradorite as the most frequent source of the Huronite. 



PETROGRAPHICAL DESCRIPTIONS. 



i. Locality. At Hudson's Bay Co.'s Post, Bear Island, Lake 

 Temagami, District of Nipissing, Ontario. (From a boulder.) 



In the hand specimen the rock is a dark green, medium textured 

 diabase in which numerous large phenocrysts of plagioclase are deve- 

 loped. Most of these porphyritic crystals are more or less rounded 

 owing to magmatic corrosion, although occasional individuals exhibit 

 tolerably sharp and perfect crystallographic boundaries. Some of the 

 crystals measure as much as three inches in diameter, but as a rule they 

 vary from one to two inches across. They have in general a greenish 

 tinge, although portions of some of the crystals show a flesh red colour. 

 Most of this plagioclase is remarkably fresh and glassy, but the cleavage 

 planes are very frequently coated with such alteration products as ser- 

 pentine and chlorite derived from the decomposing bisilicates present 

 in the matrix. The phenocrysts are often seen containing or invaded 

 by portions of the finer-grained groundmass. This matrix weathers 

 brownish or yellowish owing to the oxidation of the iron present, while 



