GEOL. VOL. I.] SMITH SANTA C ATA LIN A ISLAND. 6 1 



Under the microscope the rock is seen to be composed in 

 part of an allotriomorphic aggregate of bastite, and partly 

 of irregular areas of talc, with several small patches of 

 magnesite. Small amounts of pyrite are scattered through 

 the slide. 



The bastite occurs in plates or somewhat lath-shaped forms, 

 and is colorless or with the faintest tinge of green. The 

 mineral is non-pleochroic, even in moderately thick cleavage 

 flakes. It has a pronounced fibration parallel to the vertical 

 axis. Its extinction is characteristic of a rhombic mineral, 

 being in all cases parallel to this fibration. It has a low 

 index of refraction, and gives low interference colors, much 

 like those of feldspar. The cleavage flakes show the fibra- 

 tion which is observed in thin section. Rarely a needle of 

 pyrite is seen in the fibration. Cleavage flakes give a good 

 biaxial interference figure, and show that the plane of the 

 optical axes is at right angles to the plane of cleavage and 

 parallel to the fibration. The optical character of the min- 

 eral is negative, as determined both by the mica plate and 

 quartz wedge. The bastite is everywhere altering to talc, 

 and all stages of the process may be seen. Alteration begins 

 along the margin and along the cleavage planes, and works 

 inward. Occasionally the talc occurs as a pseudomorph 

 after the bastite, giving a parallel extinction, owing to a par- 

 allel arrangement of the fibres of the talc. Usually, how- 

 ever, the talc occurs in patches of irregular shape, and with- 

 out a definite extinction throughout an entire revolution of 

 the stage, owing to the compensatory effect of the irregu- 

 larly oriented talc fibres. 



5. ORIGIN OF THE SERPENTINES. 



No detailed petrographical study was made of the serpen- 

 tine rocks of the island, but such as was made proves them 

 to be variable in their microscopic structure, and therefore 

 different in their origin. At no point was there seen any of 

 the unaltered rock from which the serpentine was derived, 

 so that the conclusions must be drawn from the microscopic 



