56 BULLETIN OF THE 



Strata that in one place are horizontal, and in another are tilted 

 at so high an angle against the mountain, are Palaeozoic — Car- 

 boniferous at top and in chief part, but comprising some Devonian 

 and, possibly, some Silurian beds also. They rest unconformably 

 on a system of plicated crystalline schists that received their 

 contortion in pre-Devonian and probably pre-Silurian time, while 

 the disturbance which uplifted the Virgin range occurred in 

 Secondary time and produced the entire cordillera system of the 

 Oreat Basin. Prof. Whitney and Mr. King have been able to 

 refer the latter definitely to the Jurassic epoch, and they have 

 ialso discriminated a third (Tertiary) system of folds of which 

 the principal manifestations are without our field. 



The subject of the distribution of sedimentary rocks in the 

 " Great West" is quite in its infancy In most of the ranges are 

 folded more than one group of rocks, and whenever a complete 

 geological map of the Cordilleras shall have been made, it will 

 show as complicated a labyrinth as that which repi'esents the 

 arrangement of strata in the Appalachians. It is only by rudely 

 tentative lines that the few general features already recognized 

 can be represented. To begin with the lowest: there is a broad 

 area in southwestern Arizona and adjacent pai'ts of California in 

 which the ranges are built of highly metamorphic rocks — chiefly 

 crystalline schists — and granite. In these schists no fossils have 

 been found, and I shall apply to them provisionally the title 

 Azoic — without, of course, intending thereby to declare them 

 destitute of life. They may prove equivalent to the Laurentian 

 and Huronian of Canada, and they may belong, in part at least, 

 to the Silurian. At present it appears most probable that they 

 are pre-Silurian, and have never been covered by other sediments, 

 but rose above the Silurian ocean (as Newberry has already sug- 

 gested), a continent coeval with that of the Laurentian hills. 

 Northward from this Azoic area stretches a band of Silurian and 

 Devonian, broad in southern Nevada and narrow in northern, 

 marking an early axis of continental upheaval. East and west 

 it is flanked by Carboniferous, and then Trias, in belts trending 

 north and south — not regularly, indeed, but rudely symmetrical 

 about the axis of elevation in central Nevada, and these in turn 

 are succeeded both east and west by Cretaceous and later beds. 



[Mr. Gilbert illustrated the distribution of the rocks by a chart, 

 and exhibited advance sheets of the map engraved to accompany 

 the preliminary report of the expedition, then in press.] 



Dr Yan Sant, of San Francisco, explained the theory of his 

 new method of lighting gas jets by electricity, and exhibited the 

 apparatus invented by himself for that purpose. 



Mr. W. Harkness read a letter from Capt. Tupman, of the 

 British Navy, communicating the results of observations made in 

 India on the recent solar eclipse. 



