[GNEOUS FORMATIONS. 291 



The stream 'of the greatest extent after thai first described occurs on 

 the northwest slope of the mountain, li has caused a bench large enough 

 tn be indicated on the map. The eastern portion of this stream is thin and 

 very lava-like in structure, with distorted and long-drawn-out pores. Under 

 the bench its thickness increases somewhat, but probably does not reach 

 that lit' the first mass, although an exact estimate is here impossible, owing 

 tu the debris. A small knoll north of the eastern end is capped by a 

 remnant of this stream. 



On the south slope of the mountain, between the two large gulches, 

 are sections <>t' tun small streams, the western one being so exposed as to 

 give a must excellent opportunity for the study of these masses. Approach- 

 ing the outcrop from the west, on the same level, a thin sheet is first met 

 with, consisting of very porous lava. This rapidly thickens eastward, and 

 at its maximum the bodj is 50 feet thick. On the western side, where 

 the mass is about 20 feet thick, the upper surface presents a mixture of 

 scoriaceous basalt fragments with sand, and the whole structure reminds 

 • me most forcibly of the appearance of the walls of modern lava streams, 

 which often consist of fragments of the quickly cooled crust broken and 

 pushed along by the molten lava within, gathering up sand and foreign 

 matter as the movement progresses. The eastern side of this section is 

 covered by ddbris, but the mass certainly dues not connect on the surface 

 with a similar stream but a few hundred yards farther east. This latter 

 body is not well exposed. The last stream to be mentioned occurs on 

 the southeastern slope of the eastern point, and is of a size corresponding 

 with the preceding. The body is a surface flow, as is shown very clearly 

 in this case. • 



The structure of these simple lava streams has been repeatedly brought 

 out in order that no doubt may arise, owing t<> incomplete statement, of 

 their actual nature as surface flows, tor this fact is important in explaining 

 some problems in the geology of the district. 



The structure of the lower lava streams and their uniform appearance 

 at certain horizons may he affirmed as sufficient evidence of their nature as 

 surface flows. They were covered by sediments of the same character and 

 formation as those beneath them, and ir is therefore quite probable thai the 



