Ransome. ] 



Eruptive Rocks of Point Bonita. 



9' 



moreover, low. In general, they give a distinct biaxial figure, with 

 a very small axial angle, the arms of the cross merely opening out 

 for a short distance and closing again. The plane of the optic 

 axis lies parallel to the longer axis of the prism, and is, therefore, 

 perpendicular to the cleavage planes. By means of a quarter-undu- 

 lation mica-plate the mineral was ascertained to be optically nega- 

 tive. The extinction in all cases is parallel to the cleavage traces 

 and the sides of the prism, and the mineral is, therefore, ortho- 

 rhombic. 



These characters serve to establish its identity with iddingsite, as 

 described and named by Professor Lawson.and it can be referred to 

 the same system of axes, a = a, b = d, and c = c; the cleavage is thus 

 parallel to the macropinacoid, the optic axes lying in the brachypin- 

 acoid. 



In sections parallel to the cleavage planes, there is discernible, 

 with high powers, a distinct fibrous structure, parallel to the edges 

 of the prism, while in iddingsite of Carmelo Bay the fibration is 

 described as being at right angles to the prismatic axis.* This led 

 to a revision of Professor Lawson's sections of carmeloi'te, and it 

 was found that in addition to the transverse fibration, which is most 

 conspicuous in the crystals stained brown by limonite, there is dis- 

 cernible in the greener varieties, with high powers (No. 8 Hartnack), 

 another system of fibers which run parallel to the prism edges, and 

 which correspond in every way to those observed in the Point Bo- 

 nita rock. The transverse fibrous structure seems to be developed 

 or accentuated pari passu with the increase of brown coloring mat- 

 ter, and in the deep brown crystals the finer longitudinal fibers 

 become quite masked. In the Point Bonita occurrence, however, 

 the limonitic pigment is entirely absent, and there is no trace of 

 transverse fibration. 



The mineral is very fragile, owing, doubtless, to its open cleavage 

 cracks and decomposed state, and could not be separated in a suffi- 

 ciently pure state for chemical investigation. In sections from the 

 prism zone there is generally noticed an irregular transverse parting, 

 with an accompanying alteration to a green serpentinous aggregate 

 along the crack. 



* Loc. cit., p. 34. 



