from a Quartz-free Diabase. 



121 



diabase intrusive in hornblende - granitite - gneiss near the latter 

 place. I obtained from the lateritic earth lying on the mass of 

 unaltered diabase a boulder of that rock in which the central parts 

 were ideally fresh, whilst the outer parts were decomposed into 

 laterite. 1 separated these parts with great care into an outer red 

 one, apparently an ordinary ferruginous laterite, and an inner one, 

 buff-coloured with whitish spots in it, which was in actual contact 

 with the unaltered rock. When I obtained the boulder these crusts 

 were quite soft, but after a few days' exposure to the air they 

 hardened considerably. 



The diabase of the boulder and its crusts of laterite were analysed 

 by Mr. lleid and myself with the following results : — 



Diabase. Laterite. 



Inner crust. Outer crust. 



100-28 100-39 100-35 



The chemical composition of the diabase indicated that it should 

 not contain quartz, but as I was desirous of ascertaining directly 

 whether or not the rock contained any quartz I treated some of it in 

 a very finely divided state by Laufer's method (fusion with micro- 

 cosmic salt). I found by this method that the proportion of silica 

 present in the form of quartz was practically negligible — - 03 of 1 per 

 cent. The diabase is therefore practically quartz-free. 



The mineralogical compositions of the diabase and of its concentric 

 lateritic crusts may be shown as follows : — 



