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Show FEAGMENTAL VOLCANIC EOCKS. 69 ment to nine sub-groups and find the law still holding good, the probabilities 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 foregoing 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 surprise 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. FRAGMENTAL 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 Plateaus 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 fragmental accumulations are seen in the central and southern portions of the * From present knowledge I am inclined to infer that dacite is about as anomalous as quartz-propyl i te. |