462 SCIENCE PROGRESS. 



titanic acid which they contain, while those of Moriah 

 are practically free from that substance. The titaniferous 

 ores occur in the gabbro, the non-titaniferous in the gneiss 

 of the district. In Canada, Lawson has pointed out exactly 

 the same distinction between the magnetite, whether dis- 

 seminated or collected in rich bodies, of the oabbros and 

 anorthosites and of the gneiss, respectively. 



Smyth (6) has given an account of the different varieties 

 of gabbro in the north-western part of the Adirondacks. 

 One kind consists mainly of felspar and augite, the latter 

 often converted into compact hornblende, but the relative 

 proportions of these constituents vary greatly. The com- 

 monest type is very rich in the ferro-magnesian minerals, 

 but there are rapid transitions into a highly felspathic rock. 

 The felspar is in general labradorite, but in the more 

 felspathic varieties of the rock it is a more acid species, 

 with perhaps some orthoclase. Another type of gabbro is 

 hypersthene-bearing. The most interesting, however, is a 

 rock consisting of dominant felspar, with some augite, etc., 

 and showing- cataclastic structures. The felspar ranges 

 from a highly twinned plagioclase to a fibrous microperthite, 

 and from a petrographic point of view the rock shows 

 transitions from a gabbro or anorthosite to an augite-syenite. 

 The latter has 65^ per cent, of silica, and about 5 per cent, 

 each of potash and soda. In contrast with the Minnesota 

 gabbro, we notice here that, as the rock varies, the minerals 

 (at least the felspars) vary with it, indicating clearly that 

 the differentiation has here been effected prior to the 

 crystallisation of those minerals. 



Without citing other papers on the gabbros and allied 

 rocks of the North American continent, we may briefly 

 advert to two or three points of general interest brought 

 out in the works summarised above. One question of 

 considerable petrological importance relates to the so-called 

 "reaction-rims" which often surround the crystals of 

 certain minerals. These consist of one or more zones of 

 various silicates, etc., interposed usually between some 

 ferro-magnesian mineral and the plagioclase felspar. The 

 phenomenon is common in other districts of America and 



