HOLMES. J 



PETEOGEAPHIC NOTES. 57 



At present tlie only view that can be taken is that before the flow of 

 the rhyolite a passage-way had been cut through the northern ranges, 

 and that the flows of lava were such as to sustain the old divide or build 

 up a ridge of sufficient height to create a new one at the present place. 

 Such is the character of the great divide district, even after a great 

 period of erosion in which enormous valleys have been cut, that a flow 

 of lava 300 feet thick, into the Yellowstone Yalley at any point above 

 the falls, would turn the drainage of the whole lake district, together 

 with that of the Upper Yellowstone Valley, into the valley of the Snake. 



It is easy to see, therefore, what a diversified succession of events may 

 have taken place in the drainage of the Park Plateau during the vol- 

 canic period, as very slight flows of lava might have kept the drainage 

 of many thousand square miles balancing between the Atlantic and 

 Pacific. 



It seems probable, however, that since the close of the rhyolite pro- 

 ducing period there has been but little change except that of the deep- 

 ening of the drainage by erosion. The greatest changes effected by this 

 agency being the carvingof thecanon and thedrainingof the ancientlake. 

 An important area of the ancient lake has been completely drained. It 

 occupied the depression now known as Hayden's Valley. 



On the north this arm extended almost to the immediate base of 

 Mount Washburn. The accompanying map, Plate (XXXI), gives an 

 approximation to the outline of the ancient lake. 



PETEOGEAPHIC l^TOTES ON^ THE VOLCANIC EOCKS OF 

 THE YELLOWSTOl!^E PAEK. 



By Capt. C. E. Dutton. 



Washington, D. C, Jiihj 3, 1879. 



Mr. W. H. Holmes: 



My Dear Sir: I have examined the volcanic rocks you collected in 

 the Yellowstone Park and vicinity, and they are very interesting. The 

 rhyolites especially present an attractive study. They are all of the 

 most typical kind, being hyaline or vitreous, and abounding in those 

 characters which are associated or even correlated with the most siliceous 

 eruptive rocks. I understand from you that this series of rocks comes 

 partly from massiv^e sheets of ancient lava and partly from volcanic 

 conglomerates. Those from the former sources are rhyolites and basalts ; 

 those from the latter are chiefly hornblendic andesites with a few horn- 

 bleudic trachytes. The former are much younger than the latter and 

 agree as to their time relations with the general laws of suc(;ession of 

 eruptions prevailing throughout the mountain regions of the West. As 

 to the geological relations of these rocks, I am unable to say anything, 

 being insufliciently apprised of the observed facts. 



There is one featuie which seems to be shared by pretty nearly all 

 these ro(5ks, and that is the great amount of alteration which tliey have 

 undergone since their extravasation. The alterations are not only ex- 

 tensive but are somewhat peculiar. It is most conspicuous in the horn- 

 blendes, which have been altered, not, as more frequently ha[)i)ens, into 

 vjridite or magnetite, but into ])eroxide of iron, which has tlie facies of 

 hematite rather than limouite. This alteration, however, generally 



