218 



J. F. KEMP — GABBROS OF LAKE CHAMPLAIN. 



district of Quebec, and by Lawson from lake Superior ;* but in these in- 

 stances from lake Champlain the dots are nearly or quite colorless and 

 only their enormous numbers, coupled with their refractive properties, 

 lead to opacity. They are not accompanied by rods or other larger or 

 more definite forms, although they themselves are ranged in ranks par- 

 allel with the brachy-pinacoid. Figure 36, given by Lacroixf on page 

 232 of his valuable paper on " Gneiss k Pyroxene et des Roches a Wer- 

 nerite," is very similar in its general appearance, and the inclusions are 

 said to be of a faint green. The writer is inclined to regard them, as does 

 Lacroix, as fine pyroxenic dust (they are less than 0.01 mm. in diameter) 

 and hence as inclusions and not alteration products. 



YiGVRi:.!.— Thin Section {number 146) of Gabbro. 



Sliowing reaction rim of brown hornblende (H) between green augite (Py) and decomposed lab- 

 radorite {¥}. Specimen obtained near Port Henry, New York. 



The other larger minerals are light green monoclinic pyroxene, hyper- 

 sthene, grains of titaniferous magnetite, and occasionally, but not inva- 

 riably, irregular crystals of olivine. Almost always around each, of these 

 are the most beautifully developed zones, which include, in one and 

 another, brown hornblende, hypersthene, garnet, brown biotite and quartz. 

 The simplest case is shown in figure 1 . Around a crystal of pyroxene 

 (Py) a zone of brown hornblende (H) in small individuals has gathered, 



* A. 0. Lawson: Anorthosites of the Minn. Coast of Lake Superior, Bull. 8, Minn. Geol. Survey, 

 1893, p. 8. 

 t A. Lacroix : Bull, de la Soc. Min. de France, vol. xii, 1889, p. 83; see especially p. 228. 



