CLASSIFICATION OF RHYOLITIC BOCKS. 59 



andesite, to No. 12, an extremely siliceous and potassic rhyolite/' Separation of this 

 series into dacites and rhyolites is evidently largely arbitrary; hut the dacites and 

 rhyolites of Tonopah appear to be roughly comparable to those of Eureka and Washoe, 

 and as they are on the whole distinct rocks (in spite of the transitions) it is desirable 

 to have separate field names for them. For this reason it seems advisable to the 

 writer to retain the field name dacite for the less siliceous and alkalic of the dacite- 

 rhyolite rocks at Tonopah. 6 



Rhyolitic nature of loth dacites and rkyoJite*. To determine the position of the 

 Tonopah dacite-rhyolites in the system of igneous rocks the writer has compared 

 their analyses with similar analyses. As almost all comparable rocks have been 

 classed as rhyolites, this designation would apply to these rocks, and there would be 

 no distinction between the white siliceous rock of Mount Oddie and the darker rock 

 of Brougher Mountain. If the region had been mapped without strict accuracy and 

 detail, therefore all these phases would probably have been included together and 

 mapped collectively as rhyolites, and the significance of their relations would have 

 been lost sight of. 



Determination according to a quantitative classification. The word rhvolite is 

 part of the old-established classification, and its meaning is indefinite and inexact. 

 Undoubtedly the most notable attempt at an exact classification of igneous rocks is 

 that recently made by Cross, Iddings, Pirsson, and Washington/' Their own char- 

 acterization of the sj'stem is as follows: 



"This system is a chemico-mineralogical one. All igneous rocks are classified 

 on the basis of their chemical composition, and all rocks of like chemical composi- 

 tion are grouped together. The definition of the chemical composition of a rock is 

 expressed in terms of certain minerals capable of crystallizing from a magma of the 

 given chemical composition, and the expression is quantitative."'' 



a Such rooks have been called tordrillite by the writer. Am. Geologist, vol. 25, p. 230. 



("Since the classic work done in Nevada by Zirkel, Hague and Iddings, Becker, and others, some further division in 

 petrographic nomenclature has been made in rocks similar to those which they studied. Brogger has given the name 

 monzonite to granular rocks occupying an intermediate chemical position between granites and diorites. This group 

 therefore is made up of rocks which previously were classified either as granitesor diorites. Dr. F. L. Ransome has followed 

 out this idea and assigned a special name latite to extrusive rocks having a monzonitic composition. This new division is 

 made upof rocks previously classified as rhyolites, dacites, and andesites The Sierra Nevada volcanic province whose latites 

 were described by Dr Ransome is probably part of the same petrographic province as that in which Nevada lies (Spurr. 

 3. E., Jour. Geol., vol. 8, No 7, p. 638). Latites, indeed, are abundant in Nevada, and have there been described by the 

 writer; and mouzonitesare also present (Spurr, J. E . Bull U.S. Geol Survey No. 208, pp.53, 59, 73, 92, 108. 118, 122.126. Hl.lstij. 

 The latites correspond to a part of the dacites and andesites described by the earlier investigators in the region, as previously 

 pointed out by the writer (Spurr, J. E., Jour. Geol., vol 8. no. 7, p. 643). Thus a number of the dacite and andesite analyses 

 given for the Washoe and Eureka rocks would to-day be doubtless classified as latite by most petrographers. 



Under the newer nomenclature and subdivision, therefore, the rhyolitic series at Tonopah would pass with decreasing 

 silica.increasing lime, and attendant changes to&lntite rather than a docile, and this is theclassification which the writer would 

 use were the Tonopah district an independent problem. Actually, however, the correlation of these Tonopah lavas with 

 those already described at Washoe and Eureka, as well as other parts of Nevada (Spurr, J. E., Jour. Geol., vol. 8, no. 7, pp. 

 621-646), is a highly important feature of the investigation; and most of the previous work on this region has been stated 

 simply in terms of basalt, andesite, dacite, and rhyolite. Thus the writer would be compelled to reorganize completely the 

 literature of the province in order not to introduce more confusion than illumination, and this task he does not at present 

 feel able or anxious to undertake. 



< Quantitative Classification of Igneous Rocks, 1903. 



<* Washington, H. S., Chemical analyses of igneous rocks: Prof. Paper U. S. Geol. Survey, No. 14, p. 47. 



