ENDLicH.] COMPARISON OP ERUPTIVES. 259 



found overlying rhyolite, is one that certainly invites attention, but 

 cannot seriously affect the position which is generally assigned to rhy- 

 olite. 



This rock presents many varieties. At times it resembles a typical 

 porphyry, showing a solid paste with numerous crystals imbedded 

 therein, and again it will assume the form of a crystalline aggregate. 

 Whitney* cites an instance of this kind. Near Lassen's Peak there is an 

 occurrence of nevadite. "It has a resemblance to granite, so that, at 

 a distance and without close examination, it would be taken for that 

 rock." 



An important feature of rhyolite was discovered by Dr. Loew.f He 

 found that nearly all the rhyolites he examined contained either traces 

 or small quantities of nickel and cobalt. He determined it as being in 

 the form of oxide. The largest quantities he obtained were out of speci- 

 mens from 



Gila Eiver. Sierra Caliurd. 



Nickel oxide ) ooSnercent f t'^ac^- 



Cobaltoxide '. f "*"'^ percent. | o.02 per cent. 



Usually it is found only in traces, however. This fact established a 

 curious connection between the rhyolites and meteorites. Nickel and 

 cobalt may be classed among the metals of compara.tively rare occur- 

 rence. J. Lawrence Smith states| that he has made over one hundred 

 different analyses of meteoric iron, and never failed to find cobalt. It 

 is evident that in the ordinary course of a rock-analysis both cobalt and 

 nickel would most likely be lost, so that we may safely infer that the 

 rhyolites examined by Dr. Loew were not exceptional cases, but may 

 rather be regarded as expressing a usual ingredient. 



One of the most widely-distributed volcanic rocks is basalt. Fre- 

 quently we find dolerite associated with diabase. This latter is of very 

 little importance in Colorado. Both south and west, however, it has 

 been found. Mr. Gilbert mentions a sanidin-dolerite from the San Fran- 

 cisco Mountains, which evidently occurs in considerable quantities. 



All the world over rocks have been found that are referred to basalt. 

 A number of varieties have been distinguished, dependent mostly upon 

 the presence or absence of some characteristic mineral. In the most 

 ancient times basalt was evidently mistaken for the highly-prized lydite 

 or basanite. According to Ouenstedt,§ the present name is the result 

 of an error committed by a copyist during the fifteenth or sixteenth 

 century. The transition from basanites to basaltites, the old name, is 

 not a difficult one. 



In every respect is the i^osition and character of basalt so thoroughly 

 recognized that no reference need be made thereto. We have seen that 

 in Colorado large areas are covered by it, and we know of its existence 

 almost throughout the entire Western country. Its age, so far as Col- 

 orado is concerned, is readily established. In other regions, however, 

 basalt sometimes bears totally different relations to surrounding sedi- 

 mentary beds. Geikell claims that the melaphyrs in Scotland are of 

 Carboniferous age. The basaltic dikes in the granite of the Sweetwater 

 region^ are evidently not of Tertiary age, but older. Other instances 



•Geol. Saw. Calif., 1865, vol. i, p. 315. 



tExpl. aod Surv. west 100th Mer., 1875, vol. iii, p. 646. 



i Am. Jour Sci. and Arts, 1870, No. cxlvii, p. 331. 



$ Compare Ouenstedt, Handbuch der Mineralogie. 



II Geolog. Magazine, London, 1870, p. 136. 



H Compare Eep. U. S. Geol. Surv., 1870. 



