University of California. 



[Vol. i. 



the grating structure and the mesh structure. The grating struc- 

 ture is characteristic of areas of serpentine derived from enstatite 

 and diallage; in it the bands of parallel fibers are in position cor- 

 responding to the direction of the dominant cleavage in the original 

 mineral. The fibers are at right angles to the lengths of the bands. 

 The mesh structure is found in areas of serpentine derived from 

 olivine. In this structure the bands intersect somewhat irregularly, 

 but generally in rectangular positions, surrounding eye-like areas of 

 felted serpentine. The bands are frequently marked by a concen- 

 tration of limonite and magnetite along their course in both types 

 of structure. Experiments made on several thin sections of ser- 

 pentine proved that it was readily gelatinized by both hydrochloric 

 and sulphuric acid. 



The pyroxene is of two kinds, an orthorhombic variety with 

 the characters of enstatite, and a monoclinic pyroxene, determined 

 as diallage. The enstatite, which is by tar the more abundant of 

 the two, occurs in large individuals, which are very noticeable in 

 hand specimens of the rock, the brilliant cleavage faces causing 

 them to stand out prominently from the dull green of the surround- 

 ing serpentine. Though apparently porphyritic in character, these 

 individuals have in no case definite crystal boundaries, but are rem- 

 nants of an originally coarse allotriomorphic-granular rock which 

 have escaped the serpentinization affecting most of the mass. In 

 thin section it is colorless and presents one dominant cleavage 

 parallel to the brachypinacoid. The prismatic cleavage may be 

 recognized in basal sections, but is feebly developed. The inter- 

 ference colors are low, varying from pale green to yellowish gray, 

 in this respect presenting a marked contrast to the diallage found 

 in the same slide. Cleavage fragments exhibit parallel extinction, 

 and plates parallel to the pinacoidal cleavage show no optical figure 

 — features which prove its orthorhombic character. Many of the 

 enstatite crystals exhibit the effect of pressure or shearing force in 

 the bent and twisted condition of their cleavage laminae, resulting 

 in an undulatory extinction between crossed nicols. The source of 

 this stress probably lies in the increase in bulk of the rock result- 

 ing from the hydration and serpentinization of the minerals com- 

 posing it. 



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