46 



CRUISE OF THE STEAMER CORWIN. 



scales like tridymite. So far as observed, however, noue of these scales present a regular hexag- 

 onal outliue, but resemble more closely those figured by Roseubusch* thau any I have seen figured 

 elsewhere. 



The dark variety of the rock is much more compact in texture, and.bears a larger proportion 

 of inicroKcopic hornblende, which occurs in crystals of all sizes up to one-fourth of an inch in diam- 

 eter. Under the microscope it is found to contain also a much greater proportion of minute feld- 

 si)ars scattered through the groundmass. As in the lighter variety, these are short and thick, 

 being usually not more than twice as long as broad. The groundmass is much more dense, but 

 under a high magnifying power is seen to consist mainly of the same colorless uiicrolites and iron 

 ore. Little, if any, tridymite is present, and no true glass was observed. In other respects the 

 varieties seem nearly identical. 



Samples of the rock submitted to Dr. T. M. Chatard, of the Geological Survey, for chemical 

 analysis, yielded results given below : 



[I is the ligUt-colorod tridymite-bearing variety ; II, the dark variety.] 



Iguition 

 SiOi . . . . 

 TiOv.... 

 AliOs .. 



FeO .... 

 MuO... 

 CaO .... 

 MgO... 

 TjOs.... 

 Na-iO... 

 KvO ... 



The low percentage of silica in the rocks is especially interesting, and would seem to point to 

 the presence of very basic iilagioclases. To satisfactorily determine this point an attempt was 

 made to separate the feldspars from both rocks by means of the iodide of mercury and potassium 

 solution. At a specific gravity of 2.7 a considerable quantity of the powder from the light variety 

 came down, which, on examination with the microscope, proved to be nearly all feldspar, with 

 small portions of the groundmass, and included iron ores and hornblimde particles. After stand- 

 ing over night a further precipitation was observed to have taken place, which, on examination, 

 proved to be very pure feldspar. A sample of this last submitted to Trofessor Clarke for further 

 examination yielded 55.29 per cent, of silica, which is about the right proportion for labradorite. 

 Owing, however, to the somewhat crude method of separation, I do not feel at all certain that 

 ihis is the only feldspar present, and it is very probable that witli better facilities other results 

 might bo obtained. It is possible that the first portion to come down may have been auorthite, 

 as suggested by Mr. Diller in Science of January 23, 1885. 



With the dark variety of the rock no satisfactory results could be obtained, it being found 

 impossible to separate the vt-ry impure feldspar from the groundmass with any facilities at my 

 command. 



Especial thanks are due Professor Clarke and Dr. Chatard for the excellent chemical work 

 done by them in this connection. 



National Museum, February 23, 1885. 



' Mikrosoopisclie Physiographie der Miueralien, etc., p. 227. 



