STRUCTURE OF SERPENTINE. 1J5 



other biaxial varieties. To give separate niineralogical names to mere vari- 

 ations in the arrangement of tlie foils of a foliaceous mineral seems an un- 

 necessary complication of terminology, nor can I see why the presence of 

 a peculiar luster in the so-called bastite should entitle it to a separate name. 

 Lustrous labradorites are not regarded as of a different mineralogical spe- 

 cies from labradorites not possessing this interesting peculiarity. It thus 

 appears sufficient to classify the mineral characterizing the serpentinoid 

 rocks of the Coast Ranges as a distinctly biaxial serpentine. This sepa- 

 rates it from the colloid mineral of Messrs. Fouque and Michel-Ldvy and 

 from the possibly uniaxial antigorite of Mr. Eichstiidt. The necessity of a 

 division of serpentine into more than these three mineralogical varieties 

 seems to me doubtful. 



Of more interest and importance than this minute classification of va- 

 rieties is the structure resulting fi-om the grouping of adjacent microscopic 

 aggregates. Two types of such structure are known, one in serpentine, 

 produced by the decomposition of olivine and representing the net-work of 

 cracks of the parent mineral, usually emphasized by the presence of more 

 or less opaque matter. The other, called grate structure, was first studied 

 by Mr. von Drasche in Alpine serpentines, which he showed to be derived 

 from augitic and amphibolic rocks. In serpentines of tliis class the foliated 

 or fibrous mineral is arranged in narrow, somewhat sharply limited bands, 

 which are nearly straight, though often discontinuous, and cross one another 

 at high angles. The interstitial spaces are filled with less regularly dis- 

 posed material. Mr. F. Becke observed the same structure in some of the 

 Grecian serpentines. Mr. Eichstiidt has shown that in some of the Swe- 

 dish serpentines olivine decomposes into serpentine, exhibiting this grate- 

 structure, whicli consequently does not necessarily represent cleavages in a 

 parent mineral. It appears to me probable that it is at least in part con- 

 nected with a change of volume attending the decomposition of the min- 

 erals from wliicli the serpentine is derived. There can be little doubt, how- 

 ever, that in some cases the position of the grate-bars has been influenced 

 by cleavages in the original mineral. 



In the serpentines of the Coast Eanges tlie ollvinitic net-structure has 

 not been detected, nor has any olivine been found either in the serpentine or 



