32 



PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



Apatite occurs but sparingly and in minute colorless crystals, show- 

 ing but slight trace of the dusky interiors so often seen in the apatites 

 of this class of rocks. The magnetite is, as a rule, in but poorly defined 

 crystals. 



The base proper of the rock consists of an aggregate of minute color- 

 less microlites • and grains of opacite ; there is also present a very 

 weakly doubly refracting, colorless, interstitial substance, which, un- 

 der a power of 300 diameters is seen to be composed of rounded and 

 irregular imbricated scales like tridymite. So far as observed, how- 

 ever, none of these scales present a regular hexagonal outline, but re- 

 semble more closely those figured by Eosenbusch t 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 diameter. Under 

 the microscope it is found to contain also a much greater proportion of 

 minute feldspars 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 

 microlites and iron-ore. Little, if any, tridymite is present and no true 

 glass was observed. In other respects the varieties seem nearly iden- 

 tical. 



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.] 



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 

 potassium solution. At a specific gravity of 2.7 a considerable quantity 



*In a preliminary note on these rocks, published in Science of December 12, 1884, 

 the base was stated by mistake to be microfelaitic. It should have read microUtic. 

 t Mikroscopische Physiographie der Mineralien, &e., p. 227. 



