ORIGIN OF SERPENTIIs"E. 283 



niation of serpeniiue as an alteration product of amphibole and pyroxene lias 

 been not unfrequently observed and actual pseudomorpbs have been found.^ 

 It thus appears evident that a part at least of the serpentine in these 

 rocks is an alteration product of amphibole and pyroxene, and in further 

 confirmation of this hypothesis the microscope shows, in specimens of a 

 green silicious rock from the lower part of the Red amphitheater and of a 

 similar rock from the south base of Mount Lincoln, among fresh and un- 

 mistakable amphibole crystals, some in process of decomposition, whose end 

 product is serpentine, the remaining components of the rock being quartz 

 gi'ains and calcite in alternate layers. 



J. D. Dana,'- in treating of similar occurrences of serpentine in the dolo- 

 mitic limestones of Southern New York, supposes that the process of change 

 was that by a first metamorphism the uncrystallized dolomite became pene- 

 trated with tremolite, actinolite, and other magnesian silicates, and that 

 "these beds underwent a later transformation, converting the tremolite and 

 other magnesian silicates and part of the remaining dolomite into hydrous 

 magnesian silicates and mostly into serpentine." At first glance the Mosquito 

 Range phenomena seem to present a further analogy with those of Southern 

 New York in that there are presented two periods of possible metamorphism 

 (or activity of metamorphic action), viz, that following the intrusion of the 

 porphyries and diorites and that following the folding and faulting which 

 accompanied the uplift of the range. There is, however, no evidence that 

 the dynamic movement was either accompanied or directly followed by an}- 

 widespread metamorphic action. The decomposition of metallic minerals, 

 which was a metamorphic action, preceded this movement and followed the 

 eruption of porphyries. 



As to whether the serpentine has been derived entirely from amphibole 

 and pyroxene, or whether a part may have been derived directly from 

 dolomite, as suggested by Dana in regard to the New York occurrence, 

 no definitely conclusive evidence has been obtained. No opportunity was 

 off'ered for tracing the yellow rock, which would seem probably to have 



'J. Koth: Allgem. u. cbem. Geologie, pp. 123, 127, 131. Berliu, 1879. A. Lagorio: Mic. Anal. 

 Ostljaltisclier Gebiigsarten, p. 43. K. B. Hare : " Die .Serpeutiu-Masse von Reicbeusteiu" (Neues Jahr- 

 bucb, II. Bil., p. 34G. 1880.) 



-Americau Jonrnal of Science, Vol. XX, p. 32. Jnly, 1830. 



