OCCLUSION OF IGNEOUS ROCK 423 



they mark earlier stages in alteration, whose more advanced 

 condition has been reached in serpentinoids occurring southward 

 in the same belt, in Pennsylvania and Maryland. 



As to the actual mineralogical constitution of sei'pentinoids 

 in other regions, while a common view that they are but mix- 

 tures of minerals in indefinite and indeterminable stages of alter- 

 ation will probably be entirely set aside, great differences un- 

 doubtedly occur, only to be ascertained by special investiga- 

 tion in each case. The following constitution, for example, 

 has been determined ^ for a serpentinoid derived from gabbro, 

 in Northern Syria : 2 molecules serpentine, i clinochlore, 

 3 hyaline silica. In another, derived from olivine-gabbro, 

 the dark green ground-mass was found to consist of 1 5 ser- 

 pentine, 2 clinochlore, 3 amesite, i opal, 2 magnetite ; while 

 the whitish green schlieren were composed of i amesite, i clin- 

 ochlore and 13 opal. More closely allied to our own rock is 

 the "pseudo-serpentine" of Washington, recently described,- in 

 whose composition silica falls to 13.08 per cent. Its minera- 

 logical constitution was shown to approach the following fig- 

 ures : Hydromagnesite 5, chlorite 14, serpentine 20 and brucite 

 60 per cent. 



Constittitioii of the Original Igneous Rock. — The exact source 

 of the free magnesium hydrate in our serpentinoid is yet to be 

 sought. Its absence elsewhere from extensive exposures of 

 serpentinized rocks originally rich in monoclinic pyroxene (e. g.^ 

 in northwestern New York and in Canada), amphibole and talc 

 {e. g., in Massachusetts), makes improbable any connection with 

 decomposition of those minerals in this region. All observa- 

 tions also throw doubt upon the view of derivation, in this case, 

 of brucite and free silica, with or without magnesite, from the 

 breaking up of serpentine in the belt of weathering.^ Equally 

 unfounded is the assumption that masses of free silica accom- 

 panying the serpentinoids owe their secretion in all cases to the 

 process of serpentinization of ferromagnesian minerals. In these 



' Finckh, loc. cit., 107, 1 19-120. 



2 F. W. Clarke, Am. Jour. Set., (4), XV, I903, 397-398. 



=*C. R. Van Hise, Mon. U. S. Geol. Surv., No XLVII, Treatise on Metamor- 

 phism, 1904, 349-350- 



