GEOLOGY. 307 



of sandstones, which have a pretty uniform dip of from twelve 

 to fifteen decrees to the northwest. 



The observations of Gilbert in the Henrv Mountains of 

 Southeastern Utah present a peculiar and somewhat similar 

 type of intrusion. The ancient lavas there having failed to 

 penetrate the upper portions of the horizontal strata, uplift- 

 ed them for considerable areas, and, spreading out between 

 the separated strata, formed subterranean lakes, the consoli- 

 dation of which has given rise to interbedded lenticular mass- 

 es of plutonic rock, now disclosed by erosion. 



An area of over five thousand square miles of eruptive 

 rocks in Southeastern Utah is found to be made up of suc- 

 cessive flows, which do not include any sedimentary beds 

 between, and present sections of from 4000 to 5000 feet in 

 thickness. These successive eruptions, which probably ex- 

 tended over a long time, belong to the close of the Miocene 

 period. The individual flows are very numerous, and repre- 

 sent all the chief groups of eruptive rocks. It is difficult to 

 determine the openings through which these great sheets 

 were extravasated, the more so as the wide-spread later 

 eruptions have concealed the older vents. It is clear, how- 

 ever, from the irregularities of the sheets and their thick- 

 ening; and thinning-out, that there were several centres of 

 eruption. The uplifting of the region, and the great faults 

 which affect the strata, were posterior to the principal erup- 

 tions; though smaller eruptions, insignificant in amount, took 

 place during the elevation, and were apparently connected 

 with the faults. These later outbursts were not in the older 

 volcanic areas, but around their borders. 



Dutton,\vho has studied these rocks, finds the oldest erup- 

 tions of the so-called propylite followed by hornblendic an- 

 desite, confirming to this extent Yon Richthofen's observa- 

 tions as to the succession. Higher in the series, however, 

 occurs a great development of trachytes, showing much vari- 

 ety in their composition, and occasionally containing mica 

 and hornblende. With these he finds repeated intercalations 

 of dolerites, made up of triclinic feldspar with much augite 

 and magnetite. The newest eruptive rocks of the region are 

 rhyolites or quartziferous trachytes, followed by true basalts. 

 These observations are to be compared with those in the 

 Record for 1876 (p. xciv), which they confirm. 



