PEOPYLITE. 85 



though otherwise very similar to those of the andesites, seldom show even 

 a trace of the black border. Barring two or throe exceedingly local excep- 

 tions, a division of the hornblende rocks of the District into those showing 

 black-bordered crystals, and those which do not contain them, would be 

 equivalent to a separation into andesites and diorites. The assertion that 

 pro[)ylit6 is characterized by the presence of hornblendes without black 

 borders is founded on the determination of chlorite as hornblende. 



Much of the chlorite mistaken for hornblende is due to the decomposi- 

 tion of augite, but though fine pseudomorphs of this description occur in 

 the slides of the Fortieth Parallel collection, the significance of their out- 

 lines appears to have been overlooked. Some of these slides are from tyj)- 

 ical, though somewhat altered, augite-andesites. Augite also occurs in the 

 dioritic porphyries of Washoe. 



Glass inclusions, in some cases partially devitrified, seem to occur in 

 nearly all the propylitic rocks of volcanic origin, while they are of course 

 absent from the dioritic rocks, included among the propylites. The Washoe 

 andesites are somewhat unusually crystalline, and if those which have been 

 regarded as propylite ever contained any isotropic base, of which there is 

 no evidence from analogy, it is now devitrified. 



Quartz occurs to a considerable extent among the granular diorites, as 

 an original constituent. One specimen of hornblende-andesite, from an 

 area not represented in the collection of the Exploration of the Fortieth 

 Pai'aliel, however, contains a few minute quartz grains of indubitably prim- 

 itive character, and these carry fluid inclusions. Occasional fluid inclusions 

 have of late years been found in all volcanic rocks, and they do not conse- 

 quently form a conclusive point of diff"erence unless they are widely dis- 

 tributed and are present in great abundance. In some of the rocks deter- 

 mined by Professor Zirkel as quartz-propylite, the quartz appears to me to 

 be secondary. It occurs in groups of grains of difi"erent orientation, and is 

 indistinctly separated from the surrounding mass. Secondary quartz, of 

 course, frequently contains liquid inclusions. 



Value o( habitus in rock-determinations. — The mcthods cmploycd to identify the 

 propylites of Washoe with other rocks were by no means confined to mere 

 mineralogical examinations under the microscope. It is but a few years since 



