METAMORPHISM OP FRAGMENTAL VOLCANIC ROCKS. 79 



around them and the fragments contained in the latter agree with the rocks 

 remaining in the former. But there is much comjjlication and obscurity in 

 many instances arising from the fact that these eruptive centers have again 

 and again been active, the work of one epoch being overflowed and par- 

 tially masked by the extravasation and still later devastation of subsequent 

 epochs. Moreover, the loftiest points are composed of massive rocks, and 

 the positions of the conglomerates are invariably below those of the centers 

 from which they are presumed to have emanated, except in those cases 

 where the relative altitudes have been changed by relatively recent dis- 

 placement. The general problem would have been full of anomalies, how- 

 ever, were we not in a position to unravel both the complications arising 

 from vertical movements and those from the recurrence of the volcanic 

 activity. But being able to restore in imagination the displaced blocks of 

 country, and in a considerable measure to separate into periods the course 

 of volcanic activity, we find by so doing that the difficulties vanish and the 

 facts group themselves into normal relations. 



A very striking characteristic of these clastic volcanic rocks, both the 

 tufas and the conglomerates, is their great susceptibility to metamorphism. 

 Not only have the beds in many localities been thoroughly consolidated, 

 but they have undergone crystallization. Those tufas and conglomerates 

 which are of older date, and which have been buried beneath more recent 

 accumulations to considerable depths, rarely fail to show conspicuous traces 

 of alteration, and in many cases have been so profoundly modified, that for 

 a considerable time thei*e was doubt as to their true character. The gen- 

 eral tendency of this process is to convert the fragmental strata into rocks 

 having a petrographic facies and texture very closely resembling certain 

 groups of igneous rocks. When we examine the beds in situ no doubt can 

 exist for a moment that they are waterlaid strata. (See heliotypes V 

 and VI.) The hand specimens taken from beds which are extremely 

 metamorphosed might readily pass, even upon close inspection, for pieces 

 of massive eruptive rocks, were it not that the original fragments are still 

 distinguishable, partly by slight difi'erences of color, partly by slight differ- 

 ences in the degi'ee of coarseness of texture. But the matrix has become 

 very similar to the included fragments, holding the same kinds of crystals, 



