470 C. H. Warren — Alteration of Augite- llmenite Groups. 



as was noted in the case of the ore in the rhodose, was 

 observed. The grains react strongly for titanium and possess 

 a magnetic susceptibility like that of ilmenite. Alteration, as 

 will be pointed out beyond, often develops the characteristic 

 reticulate structure of ilmenite. There can be no doubt, there- 

 fore, that the ore in this rock is ilmenite and if any magnetite 

 existed as an original constituent it was very inconsiderable in 

 amount. Occasional flakes of reddish brown secondary mica 

 are present. ISTo indications of any pyroxene, other than 

 augite, or of other original ferromagnesian mineral have been 

 found. 



Chemical composition and classification. — For the purpose 

 of showing the chemical character and of classifying the rock 

 quantitatively a single chemical analysis has been made on 

 material taken from a specimen showing relatively the least 

 amount of alteration. The results are as follows : — 



Analysis of Gabbro {Hessose). 



Si0 2 45-27 



Ti0 o 2-77 



ALO q 18-30 Orthoclase -Oil sal 65-93 ^ _ 



Fe a O, 3-30 Albite -053 f^ = J]^ = 2 '°> D°salane (2) 



FeO 10-13 Anorthite -104 



MgO 4-08 Corundum '006 q or L l Germanare 



CaO 7-32 Hypersthene '068 — ™ — = < -=, , , 



Na 2 364 Olivine '064 * t>M- / U>) 



K„0 1-07 Ilmenite *034 ttt\ ■', xt -r\ ™ eo xt 



W r\ an Tv/r *•* nm K,0 + Na O 69 '53, Hessose 



MnO "86 Magnetite '021 —3 ?_— — ' 



Co,Ni tr. Apatite '008 Ca0 13 ° ( 4 ) 



P.O. 1-27 



•08 K„0 11 



? T ^ ^-^ = — = '18, Hessose (3) 



H 2 2-08 Na 2 58 v y 



100-17 

 Sp. grav. 2-958 



The rock is therefore perfelic, docalcic and presodic, its co- 

 ordinates in the quantitative system being 2,5,4,3 and may be 

 called a Hessose. It may be noted that the ferrous iron and 

 titanium are both high, and that although the potash amounts 

 to over one per cent no orthoclase has been detected with the 

 microscope. Much of the potash is probably now present in 

 the biotite and sericite. 



General megascopic character of the rock. — The entire rock 

 mass has been subjected to more or less severe dynamo- and 

 hydro-thermal metamorphism. The changes thus effected are 



