424 JULIEN 



brucitic serpentinoids, at least, the facts point rather to genesis 

 from alteration of orthorhombic pyroxene, and of olivine when 

 present, in the original eruptive rock, through dissociation of 

 this magnesium silicate into brucite and quartz, or into brucite, 

 magnesite and quartz, with more or less iron oxide, probably 

 as magnetite, in the case of bronzite or hypersthene. The re- 

 actions with bronzite, in my judgment, may be those shown in 

 the following^ formulas : 



SMg^FeSi.O,, + 9HP + O = 9Mg(0H), + i2SiO, + Fep, 

 or 



3MgFeSi,0,2 + CO^ + SH.O + O = 8Mg(OH)2 



+ MgC03 + 1^2510, 4- Fe30,. 



In these formulas it has been assumed for the bronzite from 

 the analyses, Mg : Fe :: 8 : i. A decrease in volume of the 

 resultant products of the alteration is found by calculation in 

 both these cases, approximating 37.9 and 37.6 per cent, respec- 

 tively. Van Hise has shown that derivation of the same 

 products from serpentine would involve an increase in volume 

 in the two cases of 9.8 and 13 per cent, respectively. The 

 structure of the serpentinoid, however, observed at Hoboken^ 

 testifies clearly to general contraction, producing a complicated 

 reticulation of veins (marmolite, brucite and carbonates) and 

 perhaps the jointage, in advance of the schist-seams, gliding- 

 planes and faults developed during later shearing. With the 

 general removal of carbonates and silica, this shrinkage would 

 not be offset by the increase in volume, 0.44 per cent., result- 

 ing from alteration of diopside to serpentine, or to bastite, 1.93 

 per cent. 



If my view then be accepted, the relative proportions of mag- 

 nesia in the amount of brucite and that of the group of other 

 magnesian minerals may serve for calculation of the approximate 

 proportions of bronzite and diallage in the original rock. For 

 example, from the amount of magnesia, 6.19 per cent., con- 

 tained in the brucite of the Hoboken serpentinoid, and that in 

 the other magnesian derivatives, 17.52, the original pyroxenite 



