Panache. | 



L herzolite- Serpen tine. 



167 



The process of serpentinization, before referred to, may be 

 observed in all stages of development, both as to individual crystals 

 of enstatite, and as to the mass of the rock. In the enstatite the 

 change begins along the cleavage planes, gradually extending in all 

 directions until the whole mass of the crystal is involved. The 

 structure of the enstatite is so well retained, however, that frequently 

 the change can only be detected by use of polarized light, which 

 reveals the more or less complete substitution of the serpentine 

 aggregate for the original mineral. The change is sometimes, but 

 not always, accompanied by the deposition of hydrous iron oxide 

 along the cleavage cracks, the result of which is to emphasize the 

 original structure of the enstatite, as seen in the grating of the ser- 

 pentine before mentioned. 



Diallage is present in quite a subordinate amount in the slides 

 of this rock which were studied. As, however, the diallage changes 

 to a serpentine indistinguishable from that derived from enstatite, 

 and as a large proportion of this rock has undergone serpentiniza- 

 tion, it is impossible to ascertain what was the ratio of the two 

 pyroxenes in the original rock. Diallage occurs in the irregularly 

 bounded grains of varying size. It is colorless, and may be recog- 

 nized even in ordinary light, by the perfection of the orthopinacoidal 

 cleavage. More characteristic features are the appearance of high 

 interference colors between crossed nicols, and the rough surface 

 seen in convergent light. The extinction is inclined, the maximum 

 angle on the trace of the orthopinacoidal cleavage being 38 . 

 Besides the orthopinacoidal cleavage, there is well-marked prismatic 

 cleavage, seen on basal sections. No case of intergrowth between 

 diallage and enstatite was observed, though the two minerals are 

 frequently developed side by side. The diallage exhibits the same 

 pressure effects as does the enstatite, and its decomposition into 

 serpentine was observed in various stages of completeness and offers 

 no points of difference from that of the orthorhombic pyroxene. 



Olivine is present in only a portion of the slides which were 

 prepared from this rock, and where present is in small amount. 

 The ease with which olivine is changed to serpentine is, however, 

 well known, and it may well be assumed that it would be the first 

 constituent of the rock to undergo decomposition. As much of 



