212 L. V. Pir sson — Crystallization of Enargite. 



closely accords with our best estimates of the age of the falls, 

 which will make another chapter in the lake history. 



The inferred rate of terrestrial deformation in the Niagara 

 district is 1/25 feet a century; 2 - 5 feet at the outlet of Lake 

 Ontario ; and 2 feet northeast of Lake Huron. These figures 

 may be of use in reading the future of Lake Erie. 



Applied to the Erie basin, the indicated deformation con- 

 tinuing, it appears that before Niagara Falls can have receded, 

 past the Devonian ridge near Buffalo,* the drainage of the 

 upper lakes will have been turned into the Mississippi Yalley, 

 just as the Huron waters have been turned from the old 

 Nipissing channel and Ottawa River to cascade over the falls ; 

 and thus may require 7,000 or 8,000 years. In this case the 

 future life of the lakes will be very long ; as their drainage 

 will only be effected by the excavation of a deep valley back- 

 ward from the Mississippi River into the lake basins. 



Art. XX. — On the Crystallisation of Enargite ; by 

 L. V. Pirssoist. 



The crystal form of enargite, the cuprous sulpharsenate 

 Cu 3 AsS 4 , was first described by Dauberf from Peruvian speci- 

 mens in the Krantz collection presented by Breithaupt the 

 discoverer. Dauber gives only a basal projection of the forms 

 noted by him. • Also crystals from Brixlegg in Tyrol have 

 been described by Zepharovich \\ from the Serra Famatina in 

 Argentina by vom Rath§ and from Luzon by Zettler as men- 

 tioned by vom Rath.fl None of these authors give any figures 

 of the crystals observed by them except that vom Rath gives 

 a basal projection of trillings from the Serra Famatina. 



It has been thought therefore that a description of some 

 American occurrences accompanied by figures would be useful 

 and not without interest. 



Among a suite of eruptive rocks sent to Prof. Dana by Mr. 

 R. C. Hills and collected in the Summit District, Colorado, is a 

 series taken from the Ida Mine wjiich presents in an extremely 

 interesting way, as shown in an accompanying note by Mr. 

 Hills, the method of deposition of enargite at this locality. 

 It occurs in a porphyry containing large phenoerysts of ortho- 

 clase often nearly an inch in diameter. By hot solutions these 



* These estimates will be more fully explained in a forthcoming paper upon the 

 history of Niagara Falls. 



f Pogg. Ann., xcii, p. 237, 1854. 

 % Zeit. Kryst, iii, p. 600, 1879. 

 § Zeit. Kryst., iv, 426, 1880. 

 jf Jahrb. Min., i, p. 159, ref. 1880. 



