216 J. F. KEMP — GABBROS OF LAKE CHAMPLAIN. 



cally indispensable one, is meant the abnormally feldspar-rich members 

 of the gabbro family. The anorthosites are in the extreme cases quite pure 

 masses of labradorite, but these have seldom escaped dynamic crushings 

 and now present nuclei of larger fragments surrounded by cataclastic 

 rims. The feldspar at times contains copious inclusions of dusty particles 

 which are opaque and apparently oxide of iron. In some cases pyroxene 

 crystals of minute size have been recognized. All these micro-crystals 

 are regarded as original inclusions taken up as the feldspar crystallized, 

 and the same view is held by Adams * in what seem to be similar cases 

 in the Morin district. 



Where the crushing effects have been very strongly developed the 

 rock becomes a white, marble-like variety, usually much altered. The 

 secondary product is white, gives aggregate polarization when light can 

 penetrate it, and surrounds fresh fragments. It is doubtless saussuritic 

 in nature, but, save in excejDtionally thin slides, it is practically opaque. 



The commonest mineral after the labradorite is a green, monoclinic 

 augite, although this is often scarcely more abundant than a brown horn- 

 blende, which is very widespread. In the more basic varieties forming 

 the true gabbros the former often has the extra cleavages of cliallage ; in 

 other respects it hardly merits more extended description. The horn- 

 blende is frequently in close association with the pyroxene, but there 

 seems no reason to think that the former is necessarily of secondary 

 origin. The pleochroism is brown along c and 6, yellow on a. Hyper- 

 sthene is not uncommon, but it is far less abundant than either of the 

 other bisilicates, while biotite, in the area described, is practically limited 

 to the basic gabbros. Deep pink garnets are well-nigh universal, and 

 often associated in the most intimate way with the pyroxene. The same 

 cracks pierce both minerals, and, though the line of demarcation is sharp, 

 the relation makes the observer suspect that the garnet has resulted from 

 preexisting pyroxene. In the anorthosites proper the reaction rims, cited 

 later as occurring in the basic gabbros, do not appear. Magnetite of 

 titaniferous character is known, but is not abundant in the richly felds- 

 pathic rocks. 



These anorthosites, while present to a considerable degree in the shore 

 townships, are best developed back in the mountains, and for that reason 

 they are passed with brief mention here. Besides, they are much better 

 known than the true and the olivine gabbros, which are next taken up, 

 and to which this paper is specially devoted. The chemistry of the 

 anorthosites and their component minerals has already been admirably 

 and thoroughly treated by Dr Leeds,t and to his paper Dr Julien has 



*0p. cit., pp. 435-438. 



fA. U. Leeds: Notes upon the Lithology of the Adirondacks. Thirtieth An. Rep. New Yorli State 

 Museum, 1876. 



