28 AJ^KALS I^EW YORK ACADEMY OF SCIENCES 



combined water, shows that his dried residue consisted to 97 per cent, of 

 (leweylite, H,.Mii(4Si.,()jg, without any aiitigorite. 



FORMATION OF ANTIGOIUTE DIRECTLY FROM OLIVINE BY THERMAL 



ALTERATION 



The devehjpriieiit of newly t'onued antigowte among the products of 

 weathering was indeed long ago questioned by Ebelmen and others. T. S, 

 Hunt declared its entire absence from the weathered coat over the peri- 

 dotites at Montreal, Canada, d. P. Merrill and T. II. Holland also have 

 held that it is never found as a weathering product of olivine, or as a 

 constituent of laterite. Therefore, hypotheses have been devised at the 

 other extreme, according to which the genesis of antigorite directly from 

 olivine has been effected solely in a deep-seated zone of special hydi-aiion 

 below the belt of weathering, 'i'his may have progressed as '•common 

 liydrometamorphism." at a moderate thermal temperature and depth, 

 under the iiiiiueuce of moisture permeating rocks below the ground water 

 level, such waters not favoring oxidation and containing no great amount 

 of carbonic acid. "Being an essentially deep-seated process, seriicntini- 

 zation should certainly not be referred to weathering" (\^'. Lindgren). 

 Its evidences are fctniid in the entire absence of oxidation diiriu'^- the 

 passage of olivine into antigorite (G. P. Merrill, 189!» ) ; in the gi-cater 

 production of magnetite than lienuitite from the iron oxide in fcri-jferous 

 olivine, thus pointing to the scarcity of atmospheric oxygen during the 

 hydration of that mineral into antigorite (J. IT. Pratt and J. V. Lewis,. 

 1905). 



Other writers look to a still deej)i'r zoiu; of alteration to account for 

 the high water content of antigorite. as indicating connection with oro- 

 genic processes (Posenbusch, 1901 ) : there are evidences of pressure, 

 particularly in altei-ation fi-oin aiigite, which has served as a most im- 

 portant factor in the d(\('l<i|tmen1 of antigorite (T. (J. Pxinney. 1908) ; 

 with the chai-acteristics of a dc'('])-seated process, due to waters or vapors: 

 (toming from considerable dc|itlis. oi- even constituents of the magmas at 

 the time of their intrusion, which may be distinguished as liydrometa- 

 morphism (Cf. P. Merrill. 1S99) : an alteration which may have been the 

 effect of prolonged sul)mei-gcnce in sea-water under high pressure (T. H. 

 Holland, 1899). 



The variety of peridotite "stubachite" has been attributed to 



"a post-volcanic, perhaps juK'nmatolytic process, following a period of pneu- 

 inato-hydrogenic action," " 



"E. Weinschexk : N. .Ibrb. f. Min.. 1, 226. 1895. 



