Tribune Extras Lecture and Letter Series. 



and vertical distances, that within specified areas a 

 eufficieut amount of material for the sc ile proposed is 

 gathered. The Survey was most successful in obtaining 

 the requisite topographical information over large areas 

 in Utah, Colorado, Arizona, and New-Mexico. At the 

 present time this is rapidly undergoing reduction for 

 the final atlas sheets. The moving field parties are usu- 

 ally so constituted as to bo able to subdivide, and retain 

 in each an executive, astronomical, topographical, me- 

 teorological, geological, and natural history assistant. 



Topography is one of the most important branches of 

 the work, since it is to trustworthy maps of the coun- 

 try that we must always look for the most ready and 

 certain knowledge of its general features. In this coun- 

 try it has not as yet attained the dignity of a profession. 

 It is hoped, however, that ;,t uo distant d:iy it will com- 

 mand the attention it deserves at the hands of our 

 scientific institutions. Up to the present time, the area 

 covered by the survey has been as follows: 



TABLE SHOWING THE AREA SURVEYED. 



J.siHJ. JS71. 1N73. 1873. >69->73 



N.vnda 20,400 27.2oo 6,2uO .... Oil.XoO 



':ililoruia 19.150 .... .... 19.150 



Utah .... 34.400 2,500 3li.!HiO 



Anzoua 32,400 9,900 17,500 59. ,800 



New-Mexico 31,000 31,000 



I'oloraui 21,500 21,500 



Total 213.-100 78.750 50,500 72.500 228,150 



The total com of this work has beeu a little less than 

 $225,000; approximately one dollar per square mile or 

 one-eighth of a cent per acre. 



GEOLOGICAL, WORK. 



The Geological Corps of the Survey has been pro- 

 gressively enlarged in the successive field seasons. Mr. 

 G.K.Gilbert remained u member of it during 1871-2-3. 

 In 1871 ho was assisted a portion of the Summer by Mr. 

 A. R. Marvine, who was succeeded in the following 

 years by Mr. E. E. ilowcli. In 1873 the Survey was so 

 fortunate as to secure also the services of Prof. J. J. 

 Stevenson, and in the same year Dr. O. Loew, in addi- 

 tion to his multifarious labors in other departments, 

 assisted in the geological work. 



In the season of 1871 Mr. Gilbert followed a devious 

 course in Nevada and California, starting from Carliu 

 on the Central Pacific Railway aii'l touching in uc- 

 cession the Bull Kun mining district, Battle Mountain, 

 Belmout, Hyko, and Piocliu. in Nevada; (/amp Inde- 

 pendence and 1J \-ert, Wells, in California; and Camp 

 Mohave, Arizona. From tiie last placi ho accompanied 

 the boat party up the Colorado River to Diamond Creek, 

 at "which point land travel was resumed. At the cross- 

 ing of the Colorado, near the mouth of the Grand Canon, 

 he was met by Mr. Marvine, who had just commenced 

 his geological examinations at St. Gaorge, Utah, and 

 between that point and Camp Apache the routes of the 

 two geologists intersected a numijer of times, and they 

 were enabled elleciivdy to combine their observations, 

 the chief bearing of which in this region was upon the 

 definition of the southern or south-western boundary of 

 the great Plateau System. 



All that portion of the United States west of the Plains 

 IB characterized by corrugation ; that is, the geological 

 formations uncc horizontal have lieea bent and broken 

 and thrown into ridges so as to produce a mountainous 

 country. The ridges vary greatly as to night and length, 

 but agree in a general northerly trend; so that in 

 traveling north and south, it is generally easy to follow 

 valleys, whilj in going east or west one is confronted by 

 range after range : that he must climb or go around. In 

 the lower parts of this great mountain system the slow 

 but indefatigable agencies 01 rain und stream have ac- 



cumulated go great an amount of detritus that the 

 valleys are clogged and the mountains nearly or quite 

 buried. In this way have beeu produced the great 

 desert plains of Utah, Arizona, and Southern California 

 vast seas of sand and saline clay, from the surfaces of 

 which a few half-sunken peaks jut forth as islands. 

 These intermissions of the mountainous char- 

 acter are mere concealments, not interruptions 

 of the corrugated structure ; but that struc- 

 ture is interrupted in one place oerhaps in 

 others, but in ono notably by a tract in which the 

 strata are almost undisturbed. The general surface of 

 this exceptional region lies from 6,000 to 8,000 feet above 

 the ocean and it is intersected by the celebrated canons 

 of the Colorado and its tributaries. By these gorges 

 and by other modifications, chiefly dependent on erosion, 

 it is divided into a great number of plateaus which the 

 surveys now in progress are defining and naming. The 

 geologists of these expeditions have found it convenient 

 to designate the region, considered as a geological prov- 

 ince, as the region of the Plateaus, or the Colorado 

 Plateau System. It is surrounded on all sides by areas 

 of corrugation, the ranges at the east constituting the 

 Rocky Mountaiu System proper, and those at the west 

 having been designated as the Cordilleras. At the 

 north and south these mountain areas coalesce. The 

 northern portion of the Plateau System falls within the 

 belt of country studied by the geologists of the Fortieth 

 Parallel survey and rendered accessible by the Union 

 Pacific Railroad, has become tolerably well known. The 

 definition of the southern half has been accomplished by 

 the recent Engineer explorations. 



WESTERN LIGNITES AGE OF THE ROCKY MOUNTAINS. 



Tne field of operations in 1872 comprised parts of both 



Cordillera and Plateau regions, and included their joint 

 boundary from the Wahsatch range south-westward to 

 the Colorado River. At the opening of the season Mr. 

 Gilbert crossed the Cordilleras westward from Salt Lake 

 City to the Nevada line, and returned eastward to 

 Beaver, near the Hue of separation, where he was joined 

 by Mr. Howeli, who had spent most of his time east of 

 the bounding line. Between this rendezvous and the 

 next at Toquerville which also happened to be near the 

 same line these gentlemen exchanged fields. Mr. Howeli 

 keeping to the west among the mountain ranges, and 

 Mr. Gilbert exploring the plateaus about tue bead 

 waters of the Sevier and Virgin Rivers and Kanal 

 and Paria Creeks. In returning to Salt Lake 

 City Mr. Howeli once more bore to the we.-~t 

 this time so far as to touch the min- 

 ing town of Pioche, Nevada, where a week 

 was spent in a geological examination of the locality; 

 and Mr. Gilbert, after visiting the Colorado canons at 

 Paria and Kanal Creeks, returned northward over the 

 .Sevier Plateau. One of the most interesting subjects of 

 study duviag the season was the record of an ancient 

 lake tha 1 . covered the Sevier and Great Salt Lake Des- 

 erts, ana which Mr. Gilbert refers to the glacial epoch. 

 A great deal 01 atii m ion was also given to some faults 

 and fo ('.s within the Plateau region, of far le.-s magni- 

 tude tl-an those found among the Cordilleras, but of the 

 greale^i interest, since they are the simplest elements of 

 mountain structure and their study promises to throw 

 grea; light on dynamical geology. 



In i873 the work of Prof. Stevenson, who accompanied 

 Llei t. Marsh ill, was conlined to the Rocky Mountains, 

 and was quite independent of that of the other geolo- 

 gist. His investigations led luiu to very delinito con- 

 clusions in regard to the a.i;r of the Western lignites and 

 tli age of the Rocky Mouut.iiw. *uU his notes comprise 



