306 THE POPULAR SCIENCE MONTHLY. 



definitely the imbedding material which ingulfed and destroyed 

 the living trees, and in which the petrifactions are now preserved. 

 It is, as a rule, a volcanic conglomerate, or more properly a vol- 

 canic agglomerate. Both the matrix and the imbedded particles 

 are truly volcanic, the latter varying from dust particles through 

 all sizes up to those of a ton or more in weight. That the ma- 

 terial has been accumulated under the partial influence of water 

 or liquid conditions is evident from the more or less perfect 

 stratification which generally pervades it. But the fragments are 

 too angular, brecciated, to have suffered transportation and de- 

 position as subaqueous or as ordinary river deposits. The at- 

 tractive and satisfactory explanation of conglomerate formation 

 in the Utah plateau region, as given by Captain Dutton, I do not 

 think is here applicable. 



From what is said in regard to the series of forest growths, and 

 also from the evident thinness of the layers of debris, it is seen 

 that there have been many successive sheets of the material laid 

 down at the same place. In some cases and at certain places a 

 true lava flow has spread over the surface, but the lava ledges can 

 at points be seen to shade into the brecciated layers. While not 

 believing that the great mass of breccia, covering perhaps hun- 

 dreds of square miles, has been literally ejected from volcanoes, 

 as has been held in regard to such formations, I am of the opin- 

 ion that the accumulation of it is the direct and immediate result 

 of such eruption. 



Extruded lava from any source, not being perfectly liquid, 

 would cool with an irregular surface, and terminate in precipitous 

 ledges. This unevenness of surface, combined with the original 

 slope that must have existed to permit any flow, would soon cause 

 the whole area involved to be abundantly floored with volcanic 

 fragments of all sizes. During subsequent eruptions these frag- 

 ments would be swept along by and with the liquid matter, com- 

 mingled with dense showers of ejected material, amid heavy flows 

 of water from accompanying rains and perhaps melting snows, to 

 be deposited in layers at varying distances from the centers of 

 eruption, the condition in which it is now found. Most of the 

 material of which the agglomerate is composed I believe to have 

 come by the ordinary process of weathering of previously erupted 

 rocks, and then to have been commingled with finer ejected ma- 

 terial and distributed by the floods which accompanied some if 

 not all the outflows. The interstratified beds of varying degrees 

 of fineness are the results of less tumultuous periods. 



Such explanation involves the necessity for many centers of 

 eruption in the park region, for the agglomerate is of wide ex- 

 tent, and it could not be formed at great distances from these 

 centers. 



