46 



CltUISE OF THE STEAMElt COiaVIN. 



scales like tridyniito. So far as observeil, however, none of these scales present a regular hexag- 

 onal outline, but resemble more closely those figured by Roseubusch* than any I have seen figured 

 elsewhere. 



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

 of microscopic 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- 

 spars scattered through the groundniass. 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 microlites 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 light-colored tridymite-bearing variety; II, the dark variety.] 



Ignition 

 SiO. .... 

 TiOj.... 

 AlK)s... 

 FezOj... 

 FoO... 

 MnO... 

 CaO .... 

 MpO.... 

 PjO. .... 

 Narf)..., 

 KjO ... 



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

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

 made to separate the feldspars from both rocks by means of the iodide of mercury and pota.s.sium 

 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 hornblende particles. Alter 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 Professor 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 

 this is the only feldspar present, and it is very probable that with better facilities other results 

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

 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 very 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. 



' Mikroscopiacbo Pbyaiographie der Mineralien, etc., p. 327. 



