Vol. 5] 



Lbuderback. — Br nit ode. 



365 



bonaceous matter that followed the lamination. The new feld- 

 spar as determined in several places is also albite. Throughout 

 the rook small needles are beginning to develop, the more common 

 one observed being aetinolite. Some very small colorless ones 

 also occur. 



A peculiar rock occurs towards the western part of the zone. 

 It is dark colored, aphanitic, dense and fine banded, as if from 

 the effects of original lamination. Manganese dioxide stringers 

 are common. Under the microscope it is seen to be a fine micro- 

 crystalline aggregate, consisting in part of dark patches made ur> 

 largely of a brown mineral in short minute prisms with appar- 

 ently straight extinction, negative elongation, high refractive 

 index and moderately strong double refraction. In the prepara- 

 tions at hand it was not determinable. In parts veinlets and areas 

 are numerous, carrying albite, chlorite, glaucophane, etc. 



In the hope that its origin might be indicated by the chemical 

 composition, Dr. Blasdale undertook the analysis with the follow- 

 ing results. 



SiO, 



54.51 



ALO : , 



6.55 



FeA* 



19.34 



MgO 



3.47 



CaO 



5.90 



Na.O 



5.95 



K 2 



0.23 



H 2 at 110° 



0.74 



H 2 ign 



1.82 



TiO, 



0.44 



PA 



0.30 



MnO 



0.52 





99.77 



Specific gravity 3.104. 



* Not able to effect complete decomposition of mineral for ferrous iron. 

 At least 12 per cent, is ferric. 



It is not entirely clear what type of rock this represents. The 

 alumina is remarkably low and the iron high as associated with 

 the other constituents. It has considerable similarity to the 

 analysis of amphibole given on a previous page, and appears to 

 point to considerable metasomatic alteration which conceals the 

 original nature of the rock. 



