FEAGMESTTAL VOLCANIC EOCKS. 69 



ment to nine sub-groups and find the law still holding good, the probabili- 

 ties against a fortuitous coincidence becomes thousands to one; in other 

 words, a practical certainty. It only remains to discuss the subject as a 

 question of facts and not of inferences. Do the eruptions follow this law? 

 There are certain sub-groups which have not been named in the fore- 

 going arrangement, such as quartz-propylite, dacite, phonolites, &c. As 

 regards the quartz-propylites, there appears to be a slight departure from 

 the tenor of the law. Its place is among the earliest effusions, whereas in 

 chemical constitution it lies not far from the middle of the trachytic series. 

 But the disagreement is small. Dacite does not occur in the High Plateaus, 

 and I know too little of its relations to other rocks elsewhere to offer any 

 discussion.* But all the other sub-groups, so far as observed, harmonize 

 admirably with the deduced relation, and in truth I can only express sur- 

 prise at finding not one instance of real anomaly between rocks which occur 

 in superposition, although such instances have been carefully sought for 

 during two prolonged and active seasons' work and were anticipated. 



FRAGMEIvTAL VOLCANIC EOCKS. 



Some of the most interesting lithological problems presented by the 

 volcanic products of the High Plateaus are those relating to the origin and 

 development of what may be termed the clastic igneous rocks, or rocks 

 apparently composed of fragmental materials of igneous or volcanic origin, 

 but now stratified either as so-called tufaceous deposits or as conglomerates. 

 These are exceedingly abundant in all of the great volcanic districts of the 

 world, and often enormously voluminous. How those of the High Pla- 

 teaus would compare, in respect to magnitude, with those of other regions, I 

 do not accurately know, but absolutely their bulk is a source of utter 

 astonishment. They cover nearly 2,000 square miles of area, and their 

 thickness^ ranges from a few hundred feet to nearly 2,500 feet, the average 

 being probably more than 1,200 feet. Lavas are frequently intercalated, 

 but much more frequently no intercalary lavas are seen, and in general 

 they seldom form any large proportion of the entire bulk when they occur 

 in conjunction with the clastic masses. The grander displays of these frag- 

 mental accumulations are seen in the central and southern portions of the 



* From present knowledge 1 am inclined to inter that dacite is about as anomalous as quartz-propylite. 



