50 GEOLOGY OF THE TIIGII PLATEAUS. 



rock have been intruded between the Carboniferous and Mesozoic strata, 

 hoisting the upper beds into great domes. Mr. G. K. Gilbert has studied in 

 great detail the Henry Mountains of southeastern Utah, which present this 

 singular phenomenon in perfection. This group of mountains consists of 

 five individual masses, two of which are of great magnitude, and all of 

 them have been domed up by lava rising from the depths and accumulating 

 in reservoirs several thousand feet below the surface. Each of the mount- 

 ains has a considerable number of these reservoirs and the two larger masses 

 have many of them. The lava intruded itself at various horizons and con- 

 gealed, leaving lenticular masses, which are now laid bare and admirably 

 dissected by erosion. There are no indications that any notable quantity 

 of the lava ever outflowed. To these intrusive masses Mr. Gilbert has given 

 the name of " laccolites." These are by- no means isolated instances of 

 this extraordinary origin of mountains. The Sierra Abajo on the east Avail 

 of the Colorado and a small neighboring range called El Late present the 

 same phenomenon. The Navajo Mountain at the mouth of the San Juan 

 River is similarly constructed.* Several of the Colorado ranges, according 

 to Dr. Peale, owe their structure in part to "laccolitic" intrusion. But 

 mountains on the whole are rare occurrences in the Plateau Province. The 

 uplifts there are almost wholly of the tabular form. Yet, when we come to 

 examine their structure, we find that those plateaus which are due to dis- 

 placement have a construction strikingly similar to the broad platform-ranges 

 of Colorado and to the Uintas. They are found along the western belt of 

 the Plateau Province in the Kaibabs and in still more perfect development in 

 the High Plateaus. Here the uplifts have been blocked out by the usual 

 faults and monoclinal flexures. Most of them have a single fault upon the 

 western side, inclining at a very small angle towards the east. The western 

 limit is the lifted side of the fault; the eastern limit is the thrown side of 

 the next fault. All traces of the anticlinal have vanished and the structure 

 is of the simplest possible order. In a few of these uplifts we have a block 

 between two faults or monoclinals of opposite throws. Such is the Kaibab 

 Plateau itself. But the great predominance of the faults which face the west 



*TlieJfavajo Mountain is a solitary dome-liko mass of grand dimensions npon tho very brink of 

 the Glen Cauou. The canon slices off a segment of its base, and tho spectacle of rock-work, looking at 

 it from the end of the Kaiparowits Plateau across tho gulf, is overpovveringly grand. 



