9 2 



University of California. 



[Vol. i. 



Some slides containing the freshest and clearest crystals were 

 uncovered, and treated for a few minutes with hot, dilute, sulphuric 

 acid, and then with a solution of fuchsine, when the mineral was 

 found to be deeply stained, indicating a gelatinization of the silica, 

 whereas iddingsite is described as yielding pulverulent silica when 

 dissolved as a powder in hydrochloric or sulphuric acid.* For the 

 purpose of comparing the original iddingsite with that of Point 

 Bonita under similar conditions, a typical slide of carmeloi'te was 

 subjected to the same treatment, with the result that the iddingsite 

 readily took and held the stain. Thus the last apparent difference 

 between the two minerals vanishes, and they may be considered as 

 identical. 



It is impossible to do much more than speculate upon the origin 

 of this mineral, although the probabilities, based on consideration of 

 form, seem to favor its being a pseudomorph after olivine; and it is 

 to be observed in this connection that one of the objections raised 

 by Professor Lawson against this view f may be met; for the bulk 

 analysis of the Point Bonita rock shows a notably high percentage 

 of magnesia. 



Hornblende of two varieties, green and brown, occurs in a cer- 

 tain phase of the diabase, the green nearly always being intergrown 

 with the brown, and both showing frequent intergrowths with augite. 

 Sections showing crystallographic outlines are rare, for it separated 

 at the same time as the augite, and is allotriomorphic with respect to 

 the plagioclase. The pleochroism is strong in both varieties ; in the 

 brown, a is light yellowish brown, and b and c, dark brown; in the 

 green, 8 is light yellowish green, b, yellowish green, and c, dark 

 green. The absorption formula is c > 5 > a. The maximum extinc- 

 tion angle observed was about io°, both varieties extinguishing 

 together when intergrown. The intergrowths of the two with augite 

 are very abundant (Fig. 7), resembling closely those figured by 

 Iddings.J in the rocks from Yellowstone Park. The lines separating 

 the two hornblendes from each other, and from the augite, are irreg- 



* Loc. cit., p. 31. 

 tLoc. cit., p. 35. 



J The Eruptive Rocks of Electric Peak and Sepulchre Mountain, Yellow- 

 stone National Park, Twelfth Annual Report U. S. Geol. Surv., Pis. L and LI. 



