290 



BULLETIN ' OF THE 



the overflows of the Columbia basin. Geological Ecport of the Colorado 

 River Exploring Expedition, 1861; many observations. 



G. P. Scrope. The Geology and Extinct Volcanoes of Central France, 



1858, U. 



Mcdlicott and Blanford. Geology of India, 1879, I. 299. Amjgda- 



loids are very common, especially in the upper part of the various flows. 



M. E, Wadsworth. Geology of the Iron and Copper Districts of Lake 

 Superior. Bulletin Museum Comp. Zool. Cambridge, VII. , 1880, 109- 

 113. The trap sheets are amygdaloidal at the upper surface : they 

 were first shown to be overflows by Foster and Whitney, in 1850, but 

 various opinions have since been expressed concerning them. 



In the Western Territories, lava overflows have been very common, 

 and have covered vast extents of country. They are mentioned in 

 nearly all the Western Survey Ileports, but their overflow origin is as 

 a rule so evident that few special descriptions are given of the characters 

 that serve our needs of comparison. The following may be referred to 

 from among many others : King, 40th Parallel Survey, I. ch. vii. sec. v. 

 Geol. Survey of the Territories, 1874, 172 (Pealo) ; 1875, 145 (Endlich). 

 C. E. Button, Geology of the High Plateaus of Utah, 1880, eh. iii. 



The above means of distinction between intrusions and overflows are 

 almost self-evident ; but they have seldom been definitely stated in con- 

 nection with our Triassic traps, and have often been entirely overlooked. 

 The earlier studies of the traps were made when the controversy between 

 the Vulcanists and Neptunists was still unsettled; it was sufficient then 

 to show that the traps were igneous, not aqueous. But later than this, 

 and down even to recent dates, observations on the traps seldom give 

 secure basis for statement of their mode of origin, for the essential and 

 critical points for observation have been very generally neglected. In 

 many cases the older observers gave no special attention to this physi- 

 cal phase of ^the question ; their work being directed to some other of 

 the many dilTicidtics that the Triassic rocks present; and hence from 

 their accounts it is impossible to discover how the trap is related to the 

 adjoining rocks. Even Percival's remarkably painstaking and accurate 

 description of the Connecticut traps is often indeterminate as to their 

 origin ; it is greatly to be regretted that he had not better opportunity 

 to publish what he had learned, in addition to simple descriptions. 



The rare exposure of contact lines is extremely provoking : in the 

 Connecticut valley alone their combined length must amount to many 

 hundred miles, but so universal is the drift and talus covering, that one 

 seldom finds more than a few feet of "junction'* exposed. And this 



^in 



