56 GEOLOGY OF THE HIGH PLATEAUS. 



canic rocks, and at least three-fourths of all the important sub-groups have 

 here their representatives The clastic derivatives are displayed in variety 

 and volume truly extraordinary, commanding as much attention as the 

 massive rocks and presenting some highly interesting problems. It would 

 be impossible, within the limits of a single chapter, to present a good 

 synopsis of these facts with a discussion sufficiently extended (and at the 

 same time precise) to make them intelligible. Since the greater part of 

 the individual phenomena described in this work consists of those which 

 belong to the volcanic category, and since no symmetrical grouping of 

 their entire array has suggested itself to my mind, it will be practicable 

 to set forth here only those few facts of a high degree of generality which 

 appear to be applicable to the entire district. In those chapters of this 

 book which are devoted to the description in detail of the component 

 members of the High Plateaus, such facts as seem to be instructive will be 

 adverted to, together with such of their relations as have been satisfactorily 

 ascertained. 



The initial epochs and conditions of the eruptive activity of the High 

 Plateaus are obscure. The oldest observed rocks having an eruptive origin 

 are tufas. It is presumable, however, that tufas, especially such as are 

 here found, are never erupted alone, nor wholly in the fragmentary or pul- 

 verulent form, but are in part the concomitants of lava floods, and in far 

 greater part the results of the degradation of volcanic rocks. The tufas of 

 this district are stratified water-laid rocks of arenaceous texture, sometimes 

 marly or even shaly; their materials being derived almost entirely from the 

 decay of lavas. Some of these tufaceous beds are metamorphosed, and the 

 highly suggestive and interesting fact is there presented that the product of 

 this metamorphism is a rock having the essential lithologic characters of a 

 lava.* The rocks from which these ancient tufas were derived are not known. 

 An abundance of old lavas lie in their vicinity, but always on top of them. 

 There is, however, one instance in the great gorge near Monroe where a 

 propylitic mass appears to pass under some of these tufas, but owing to the 

 complications of faulting there may be a'mistake about it. Whether the 

 lava sheets which yielded by their decay the clastic materials of these 



* See Chapter XI, where this remarkable phcnomcnou is described and discussed. 



