20 THREE GEOLOGICAL PROVINCES. 



pheric degradation. Where such a cone exists, still having a well defined 

 crater, its condition testifies to the lateness of its origin, and all the facts 

 relating to the sheet of .lava on which it rests fully corroborate the conclu- 

 sion. From such evidence we are able to infer the recency of ,much of the 

 volcanic activity in the three provinces. If the human history of America 

 could be carried back to as early a date as it has been in Asia, it cannot be 

 doubted that the earlier chapters of that history would be replete with the 

 accounts of volcanic fires. 



XL HENRY MOUNTAIN STRUCTURE. 



Sometimes we find the sedimentary strata displaced by a quaquiversal 

 upheaval and the same fractured, and through these fractures floods of lava 

 have poured, and these may lie in patches about the flanks of the mount- 

 ains, or stand in dikes where the walls of the crevice have been swept away 

 by denudation. In the Henry Mountains we have a fine illustration of this 

 type of structure. These mountains have been studied by Mr. Gilbert 

 during the past season, and in his preliminary report he says : " The erup- 

 tions of the Henry Mountains are of a character entirely novel to me, and 

 they were studied with an interest stimulated by surprise. A description of 

 a single one, though it will not stand for all, will serve to. illustrate the type. 

 . Mount Ellsworth is round, and its base is six or eight miles broad. The 

 strata of the plain about it tire horizontal on every side. Near the 

 mountain the level strata become slightly inclined, rising from all sides 

 toward the mountain. At its base the dip steadily increases until on the 

 steep flanks it reaches a maximum of forty-five degrees. Then it begins to 

 diminish, and the strata arch over the crest in a complete dome. But the 

 top of the dome has cracked open, and tapering fissures have run out 

 to the flanks, and they have been filled with molten rock, which has con- 

 gealed and formed dikes. Moreover, the curving strata of sandstone and 

 shale have in places cleaved apart and admitted sheets of lava- between 

 them. So the mountain is a dome or bubble of sedimentary rocks witli an 

 eruptive core, with a system of radial dikes, and with a system of dikes in- 

 terleaved with the strata. It is a mountain of uplifted strata, distended Jind 

 permeated by eruptive rock." 



