1891.] NEW YORK ACADEMY OF SCIENCES. 21 



In I all the iron was estimated as Fe„0.^. The excess of potash 

 over soda in I and II is exceptional. While the rather abundant 

 biotite of I in part accounts for it, the amount of this mineral is 

 hardly enough to furnish it all. The silica is also lower than in 

 the Triassic diabases, which afford about 50-53 per cent. 



Olivine appears in a few dikes fresh enough to recoirnize, and an 

 alteration-product is shown by a great many more which may have 

 resulted from it. In other respects the olivine diabases are not 

 different from those without this mineral. 



Diabase, including olivine diabase, is an extremely common dike 

 rock in the Archaean rocks of Canada and the northern United 

 States, and it is probable that most of those simply recorded as trap 

 belong also with this species. 



The Camptonites. 



The name camptonite was originally employed by Rosenbusch 

 to designate those dike rocks which consist of hornblende and plagio- 

 clase, but varying amounts of augite are also often present. They 

 lack the ophitic structure of diabase in that the hornblende or augite 

 is prevailingly idiomorphic. The diabases of dikes thus pass into 

 them by the idiomorphic development of the dark silicates. In the 

 Lake Champlain region there are dikes of both the augitic and the 

 hornblendic variety. We have employed the name augite-cam[»- 

 tonite when wishing to make a distinction. 



The camptonites consist of brown basaltic hornblende, augite, 

 plagioclase, magnetite, and occasionally a. little intermingled glassy 

 matter. The minerals are markedly panidiomorphic and the large 

 hornblendes and augites give at times a porphyritic character. The 

 hornblende is the most conspicuous and attractive component. It 

 is strongly pleochroic, brown to yellow, and, in distinction from 

 the augite, shows no zonal structure. A small second generation 

 consists of minute acicular crystals, which are a miniature repro- 

 duction of the larger forms. The augite likewise forms two genera- 

 tions. The older and larger consists of zonal prismatic crystals with 

 dark green cores and light yellow rims, which may differ 10° in 

 extinction. The second generation are minute and acicular. The 

 plagioclase is less perfectly developed than the bisilicates. Several 

 camptonites contain abundant olivine. 



The camptonites all contain less than 45 per cent. SiO,, in this 

 region, but in other respects present great variability. They do 

 not afford more than (! per cent, of alkalies, with soda usually in 

 excess. The following table illustrates the range of composition : — 



