Makch 24, 1899.] 



SCIENCE. 



429 



over broad fields and thrown out under 

 varying conditions. Frequently these basic 

 breccias present a rough and ropy surface, 

 like ordiuny scoria irregularly heaped to- 

 gether, but the bulk of it indicates indistinct 

 bedding. A tumultuous heaping-up of ag- 

 glomerate by explosive action characterizes 

 this breccia, which not infrequently carries 

 andesitic and basaltic bowlders measuring 

 6 and 6 feet in length and often double that 

 size. In one or two localities huge bowlders 

 of crystalline gneisses and schists are also 

 embedded in the lavas. 



Scattered over the area occur the thin in- 

 terbedded flows, apparently poured forth 

 from numerous fissures and vents. These 

 flows increased in frequency and thickness 

 until finally massive outflows of basalt cov- 

 ered a considerable portion of the earlier 

 series of breccias. Over how large a field 

 they at one time may have extended cannot 

 now be told, erosion having certainly re- 

 moved them from large tracts, but they may 

 never have been spread over extensive re- 

 gions. It is somewhat curious that this 

 continuous broad field of basalt has a north- 

 west-and-southeast trend and stretches 

 obliquely across the summit of the range 

 from Mirror Plateau to Needle Mountain, 

 whereas the body of the breccia in general 

 has a north-and-south trend. The basalts 

 lie upon the uneven surfaces of the breccia 

 and occur piled up in a succession of flows, 

 which in places near their sources h4ve at- 

 tained an aggregate thickness of 1,500 feet, 

 although over large ai-eas they measure 

 about ] ,000 feet, thinning out to a few hun- 

 dred, while in certain places they appear to 

 be wanting. Individual sheets range in 

 thickness from 5 to 50 feet without showing 

 any material change in the physical charac- 

 ters of successive flows. The greatest accu- 

 mulation of flows appears to be along the 

 trend of the basaltic body, thinning out 

 both to the northeast and to the southwest, 

 indicating that the eruptions had followed 



a fissure or system of fissures. Of course, 

 this can be said only in a general way, as 

 basaltic outflows may occur anywhere along 

 the range. As regards mineral composi- 

 tion, they are usually fine grained, with but 

 few well-developed megascopic constituents, 

 mainly augite, olivine and plagioclase. In 

 chemical composition they show within re- 

 stricted limits considerable variation, with 

 accompanying changes in mineral develop- 

 ment, analyses determining a large amount 

 of the alkalies and a correspondingly low 

 percentage of silica. ISTumbers of these 

 flows have built up, from vents, rounded 

 bosses of basaltic rocks characterized by a 

 development of orthoclase, in several in- 

 stances associated with leucite. They are 

 the extrusive equivalents of intrusive rocks, 

 designated as absarokites in distinction 

 from normal basalts. Eeference will be 

 made to them later, in speaking of certain 

 intrusive masses. So far as our present 

 knowledge goes, they belong chiefly to this 

 period of eruptions. Many of these indi- 

 vidual sheets stretch out for long distances, 

 but others show great lack of continuity, 

 thinning and thickening in different direc- 

 tions and often overlapping one another, in- 

 dicating numerous sources of eruption and 

 varying force and duration of flows. 



In their topograpic configuration the ba- 

 salts stand out in marked contrast to the 

 loosely compacted breccias, owing to great 

 unifoi-mity of flows and to differences in 

 weathering. To these basalts the name early 

 basalt sheets has been given, and they are 

 here treated as a geological unit, since they 

 mark a distinct period in the history of 

 volcanic eruption. It is quite possible, and 

 even probable, that they covered this entire 

 region and were subsequently removed by 

 erosion, but of this there is no direct evi- 

 dence. If they did, the country must at 

 one time have presented a gloomy, sombre 

 field of basalt, poured forth in a molten 

 condition after a long period of fragmental 



