GEOLOGY. 249 



Next to active volcanoes in significance to systematic geologists, and 

 in interest and iustructiveness to laymen, come those recently extinct; 

 and during tlie past two years such a volcano lias been investigated by 

 Diller. This volcano is perhaps the most recent within the United 

 States. Its crater, overlooking Snag Lake 10 miles northeast of Lassen 

 Peak in northern California, is indeed cold, but the lapilli and scoria of 

 which it is composed have been scarcely affected by tlie action of the 

 elements, and its slopes are as high as such material will maintain ; 

 the lava sheet which flowed from it, and which by damming a small 

 creek formed Snag Lake, is scarcely discolored by weathering'; the vol- 

 canic ashes which it vomited yet lie as they fell, little touched by the 

 winter rains or the summer thunder showers; the stumps of trees 

 smothered by the ashes and charred by the lava have not yet decayed; 

 and ill! the phenomena indicate that this debris-swatlied volcano was 

 in active operation but a few scores or at most a few hundreds of years 

 ago.* 



This, although probably the youngest of the series, is only one of 

 those volcanoes which, beginning in early Tertiary time and culmi- 

 nating in activity about the Pliocene or early Pleistocene, flooded 

 hundreds of thousands of square miles of the Pacific sloj^e with lava 

 sheets the most extensive of the globe. Most of the orifices from 

 which these lavas welled have been obliterated or buried by geologic 

 changes; but one of them remains, and is of such vast dimensions 

 that the water with which it is partly filled forms the deepest lake on 

 the continent. This " Crater Lake," which lies in the heart of the Cas- 

 cade range in southern Oregon, is 2,000 feet deep, and the bounding 

 walls rise 900 to 2,000 feet above its level. It was explored by Button 

 in 1880. t 



Kilauea represents active vulcanism ; Snag Lake represents the effects 

 of vulcanism yet untouched b^' time; Crater Lake is a mighty volcano 

 long dead and already mouldering into dust; the older lava slieets and 

 degraded craters of Oregon and northern California contain a record 

 of vulcanism partly eflaced by the ever operative agencies of deg- 

 adation; but there is a still later stage in the obliteration of the vol- 

 canic record which has been brought to light within two years: About 

 Mount Taylor in northern New Mexico there is an extensive area now oc- 

 cupied chiefly by sedimentary rocks of Mesozoic age, which have evi- 

 dently suffered great degradation duringCenozoic time; and through the 

 strata project cylindrical masses of basalt, sometimes large enough to 

 be dignified by the name of mountains, which evidently rei)resent the 

 contents of the pipes or ducts through which lava was extruded during 

 past ages, probably in sufficient volume to sheet vast areas with lava, as 

 southern Oregon was sheeted during the later epoch ; but these lava 

 sheets have been completely denuded over hundreds of scjuare miles, 



*Ani. Jo»rn. Sci., 1888, Sd series, vol. xxxili, pp. 45-50. 

 t Science, vol. VII, p. 179. 



