66 



SCIENCE 



[Vol. IV., No. 76. 



lake stood at the six hundred foot level a long 

 time, making a very distinct terrace (Provo 

 terrace) . Then by change of climate it lost its 

 outlet, and dried away to its present condition. 

 Mr. Gilbert correlates these changes with the 

 first glacial, the inter-glacial, the second gla- 

 cial, and the post-glacial periods. Other \erj 

 important facts brought out by Mr. Gilbert are 

 those connected with recent orographic move- 

 ments. The floor of Utah basin has recently 

 moved, and is probably still moving. The 

 movement is unequal. The floor is warping. 

 The great fault on the west side of the Wah- 

 satch has recently slipped, and will probabl} r 

 again slip. The Wahsatch Range has grown 

 very recently, and is probably still growing. 

 Such slips produce earthquakes. The great 

 Inyo earthquake of 1872 was produced by a 

 slipping of the great fault on the east side 

 of the Sierra, as was first pointed out by 

 LeConte. 1 



Mr. Russell's studies of Lake Lahontan en- 

 tirely confirm the conclusions of Mr. Gilbert 

 as to climatic changes. This lake also increased 

 from mere residues to a level of five hundred 

 feet above Pyramid Lake. Its shells show that 

 it was at this time fresh; then it dried away 

 completely, burying its salts, if any, beneath 

 pla} r a deposits. Then it rose again to the five 

 hundred and thirty foot level (it was then also 

 fresh) ; it again dried away to the present 

 residues. Its terraces are traceable all around : 

 it never found an outlet. 



Mr. Russell continues the observations, com- 

 menced by King, on the remarkable deposits of 

 this ancient lake. He divides these deposits 

 into three kinds, which were made at different 

 stages of the lake. In its first great rise, it 

 deposited a hard, smooth, incrusting lime car- 

 bonate (lithoid deposit) . At the one hundred 

 foot level and downwards, it deposited what 

 Mr. King calls thinolite, and which he regards 

 as a pseudomorph after gaylussite. In the 

 last and greatest rise, it deposited the dendritic 

 tufa. Mr. Russell calls attention to the fact, 

 that, if thinolite be a pseudomorph after gay- 

 lussite, it is difficult to understand what be- 

 came of the enormous quantity of soda ; for 

 the lake never found an outlet. This problem 

 is 5 7 et unsolved. 



It is seen, then, that Lake Lahontan , like Lake 

 Bonneville, shows two wet periods separated by 

 a dry period, and probably two glacial periods 

 separated by an interglacial period. Evi- 

 dences of recent orographic movements are 

 noted here also. Nearly all the basin ranges 

 are formed by the tilting of long north and south 



1 Amer. journ. sc, vol. xvi. p. 101, 1878. 



crust-blocks, each block being dropped on one 

 side, and lifted on the other. The faults thus 

 produced have been slipped very recently, and 

 are probably still slipping. The mountains are 

 still growing. 



The three memoirs — of Emmons on the 

 geology of Leadville, of Becker on the Corn- 

 stock lode, and of Irving on the copper- 

 bearing rocks of Lake Superior — all throw 

 light on the genesis of ore-deposits. 



In Mr. Emmons's admirable memoir we have 

 a clear scientific account (the first ever given) 

 of the wonderful argentiferous lead-deposits of 

 Leadville. In this region the mountain system 

 separating the plains from the plateau region 

 consists of three ranges ; viz., the Colorado, the 

 Park, and the Sawatch. The first two are sepa- 

 rated b}* the Parks ; the last two, by the valle}' 

 of the Arkansas River. The Park Range, in 

 the vicinity of Leadville, is called the Mos- 

 quito Range. On the western slope of the 

 Mosquito Range, or eastern side of Arkansas 

 valley, is situated Leadville. The Mosquito 

 Range consists of crumpled and faulted strata, 

 from the Cambrian to carboniferous inclusive. 

 The ore-deposits are in the carboniferous. 

 Between the carboniferous strata are thick in- 

 tercalary beds of porphyry, which have been 

 forced between the separated strata without 

 appearing on the surface. This irruption took 

 place during the cretaceous. The whole series, 

 both sedimentary and intercalary-igneous, was 

 then folded and faulted. The mode of occur- 

 rence of the ore shows conclusively that it was 

 deposited from solution in percolating water. 

 The ore occurs in a gangue of oxides of iron 

 and manganese, mixed with clay, in cavities 

 and channels in the limestone. The limestone 

 was dissolved and the ore substituted b}*- the 

 same water, the clay being the residuum of 

 the dissolved limestone. The ore was origi- 

 nally disseminated in the porphyr}^, and thence 

 leached out, and carried downward into the 

 limestone. The original form was sulphides ; 

 but in many cases this has been subsequently 

 changed into carbonates, chlorides, etc. It is 

 seen, then, that these are not true fissure- veins, 

 but deposits in irregular water-channels some- 

 what like the lead ores of Illinois, and like 

 these latter, also, the}' occur in carboniferous 

 limestone. 



These important conclusions of Mr. Emmons 

 in regard to the genesis of ore-deposits are 

 substantially confirmed by Mr. Becker's study 

 of the Comstock lode. This grandest of all 

 lodes is, however, a true fissure-vein. We note 

 only the most important conclusions of this 

 careful memoir. 



