290 ANNUAL KECORD OF SCIENCE AND INDUSTRY. 



whole of Southern Utah and Northern Arizona, and which 

 led him to call it the Plateau Province. The tables are 

 cut from the platforms of the valleys by immense faults, and 

 uplifted 2000 to 5500 feet above the valley plains, present- 

 ing nearly vertical walls fringed at their bases by rugged 

 foot-hills. The plateaus are composed of thick beds of igne- 

 ous rocks, well stratified and nearly horizontal ; the foot-hills 

 on the contrary are composed of beds much broken and dis- 

 turbed, and intermixed with lavas. The southern portions 

 are overlaid by a conglomerate, which commences near the 

 middle of the region with a thickness of at least 2300 feet, 

 and diminishes southward to 700 or 800 feet. It is composed 

 wholly of igneous fragments, often of great size, inclosed in 

 a matrix of sand and clay. In the southern portion it is un- 

 derlaid by red sandstone and white marl of Tertiary Age, in 

 the middle portion by rocks of the porphyrite class. The 

 northern and higher portion consists of well-stratified rocks, 

 having the mineral characters in some places of trachyte, in 

 others of rhyolite, but with a structural habit, a texture and 

 general mode of occurrence very unusual in these kinds of 

 rock. Lava beds occur abundantly, but so far as observed 

 are restricted to the foot-hills and valleys, and have evident- 

 ly originated from the vicinity of the great faults. Captain 

 Dutton inclines to the opinion that the stratified tabular 

 masses were metamorphosed in situ from sedimentary beds. 

 This inference is founded, first, upon the striking similarity 

 in the structure of the igneous strata to that of the adjoining 

 sedimentaries a similarity in many cases exact even to 

 small details; second, upon the absence of volcanic structure 

 in the arrangement of the beds, and in their texture ; third, 

 upon the abundant occurrence of rocks which, when serially 

 arranged, exhibit many stages of a progressive metamorpho- 

 sis; also the occurrence of many large masses which can 

 not be classified, except upon the assumption that they are 

 imperfectly metamorphosed sedimentaries ; fourth, the ad- 

 mission that they are eruptive renders the general problems 

 of structure obscure, while if they are metamorphic the struct- 

 ure is a problem no longer, but merely a repetition of well- 

 known features occurring every where in the country round 

 about. The metamorphic origin of some porphyries has long 

 been conceded, but the trachytes and rhyolites have always 



