SEQUENCE OF LAVAS. 253 



of contact, and at several localities near the outer edge of the andesitic body, 

 notably just east of the town of Eureka. 



Now, the relationship as regards age between the basalts and rhyolites 

 is placed beyond all question, numerous dikes of the former cutting the 

 latter both along the Pinto fault and in the pumice basin southwest of Rich- 

 mond Mountain. Hornitos Cone, about 400 feet in height, an isolated hill 

 rising abruptly out of the basin, is an excellent instance of the cutting of 

 rhyolite by basalt dikes. The cone is composed of light colored pumices, 

 broken through and ribbed on all sides by black basaltic dikes, which have 

 altered the siliceous rocks all along the lines of contact. Crater Cone, on 

 the east side of Richmond Mountain, affords an equally good example of 

 the relative position of the two rocks, the basaltic lavas which here form 

 the Cone flowing for long distances over the earlier pumiceous beds. Mag- 

 pie Hill, near the entrance to Rescue Canyon, affords still another equally 

 as good an illustration of the relative position of the two rocks. 



If the pyroxene-andesite overflows preceded the rhyolite it would 

 hardly have been possible under the conditions of eruption for them not to 

 have broken out along some of the hornblende-andesite centers before the 

 appearance of the rhyolites. Again, if the rhyolites followed the pyroxene- 

 andesite there should be found some field evidences of such eruptions 

 between the pyroxene-andesite and basalt, whereas, on the contrary, there 

 exists not the slightest evidence of an overflow of acidic lava intervening 

 between the closely related basic lavas. It has already been pointed out 

 that the acidic lavas hold the same close relationship to each other. 



Field observations clearly show that the order of succession of these 

 natural groups into which the lavas have been divided was as follows : 

 First, that the hornblende-andesite was the earliest of all the erupted 

 material ; second, that the homblende-mica-andesite followed the hornblende- 

 andesite; third, that the dacite followed the hornblende-mi ca-andesite; 

 fourth, that the rhyolite closely followed the dacite; fifth, that the pyroxene- 

 andesite succeeded the rhyolite; sixth, that the basalt was the most recent 

 of all volcanic products. 



TWO Magmas of Eruption. A study in the field of the geological distribu- 

 tion and mode of occurrence of the igneous rocks, shows that they all belong 



