DIABASE HILLS. 811 



centres of the present ridges. There is little evidence of much steam accom- 

 panying these eruptions, vesicular basalts and the larger cavernous open- 

 ings resulting from the expansion of considerable volumes of vapor being 

 quite rare. Both physically, and mineralogically, these basalts present much 

 the same characteristics over the entire region. Black is the prevailing 

 color. They are mostly so fine-grained as to defy mineralogical determina- 

 tion by the unaided eye, yet in sun-light are brilliant with minute faces of 

 feldspar and augite. They have a decidedly resinous lustre, and break under 

 the hammer with a conchoidal fracture and sharp edges, characteristic of the 

 half-glassy amorphous base varieties. Under the microscope, they are readily 

 shown to be normal basalts made up of plagioclase and augite, associated 

 with magnetite, varying quantities of olivine, and some apatite. Beautiful 

 specimens of clear botryoidal hyalite are frequently found as incrustations 

 upon the joints and fissures of the basalt. 



Diabase Hills. — About 6 miles from Wadsworth, along the west base 

 of the Truckee Range, and about 300 feet above the upper limit of the 

 Truckee Pliocene beds, occur two considerable hills of a light-gray diabase 

 completely surrounded by black basalt. The rock is quite uniform in char- 

 acter, and presents a fine-grained crystalline appearance. It has to the feel 

 almost the roughness of trachyte, but, upon microscopic examination, is 

 found to be a true diabase, consisting of striated plagioclase, brownish-green 

 augite, olivine, which is more or less altered into yellowish-brown serpentin- 

 ous matter, some black grains, probably magnetic iron, and many colorless 

 microlites, partly referred to apatite and partly to feldspar. No quartz was 

 detected. Between the pale-gray diabases and the distinctly overlying black 

 basalts, there is no possibility of confusion. Although microscopically com- 

 posed of the same mineral ingredients, the diabase is entirely crystalline 

 and wanting in groundmass, while the surrounding basalts are richly 

 charged with an amorphous glassy base, a microscopical distinction which 

 has produced striking differences in modes of weathering between the two 

 rocks. Moreover, the older rock has suffered much more from erosion, and 

 is more cut up by ravines and depressions, which at their base are now 

 filled by the flows of basalt, showing conclusively their later age. 



Although, in general, compact and having the superficial habit of gray 



