GEOLOGY. ■ 339 



of the succeeding" deposits were laid down in shallow seas near tide- 

 level, thus indicating a gradual subsidence from Carboniferous time, 

 going on ^jan passu with sedimentation. The Cretaceous sea, which 

 marked the close of this order of things, appears to have extended from 

 the Gulf of Mexico to the Pacific, with the exception of narrow land- 

 areas. At the close of the period, great movements took place in this 

 area, followed by erosion, which allowed Eocene fresh- water beds to be 

 deposited in some places over Jurassic strata to the thickness in parts, 

 as we have seen, of 5,000 feet. It is difficult to separate the succeeding 

 Miocene from the later Eocene, and we have here no evidence of Plio- 

 cene deposits. These were times of gradual emergence. At the close 

 of the Miocene was an uplift of 2,000 or 3,000 feet, and at the close of 

 the Pliocene a still greater one of 3,000 or 4,000 feet. In these later 

 Tertiary times came the great nortli and south faults of the Plateau 

 region, already described by Powell, producing displacements of many 

 thousand feet, and bringing in one case the Eocene against the Carbon- 

 iferous. 



The great and widely- spread volcanic activities of the region began in 

 Miocene and continued until recent times. The first erosion of the Grand 

 CaQon took place at the close of the Cretaceous, or a little later. The 

 process went on rapidly through the subsequent times of elevation, but 

 has now reached g, period of comparative quiescence. The glacial age, 

 succeeding the Pliocene, was, according to Dutton, here marked by 

 pluvial rather than glacial action. 



J. J. Stevenson, in a report on the geological survey under Captain 

 Wheeler, has given details of the geology of parts of southern Colorado 

 and New Mexico, along a continuation of the Sangre de Cristo Eange, of 

 which he notices the Eozoic rocks, before designated by Hunt as in 

 part, at least, Laurentian. Upon these, to the eastward, rest Carbonif- 

 erous strata, while farther east Cretaceous strata repose either directly 

 upon the latter or upon the Eozoic, together with some beds which may 

 be Jura-Trias, and elsewhere Tertiary rocks. Stevenson discusses at 

 some length the relations of the Laramie group, and gives new reasons 

 for believing it to be true Upper Cretaceous. 



TRIAS OF EASTERN NORTH AMERICA. 



W. M. Davis has studied the Trias of the Atlantic border, with its 

 included trappean (diabasic) rocks, as seen in the Connecticut Valley 

 and in New Jersey. These igneous rocks, according to him, occur in 

 three different relations to the associated sandstone : (1) as dikes cut- 

 ting the strata, (2) as intruded sheets, often of great extent and thick- 

 ness, lying in nearly all cases conformably between the layers of sedi- 

 mentary rock ; (3) as overflowed sheets, equal in extent and thickness 

 to the last, but poured out at the surface during the formation of the 

 sandstones. Examples of the dikes are seen near New Haven, Conn., 

 from 100 to 200 feet thick, with a transverse columnar structure. These 



