Maech 24, 1899.] 



SCIENCE. 



439 



ing to indicate the characteristic slopes of a 

 great volcano. 



AVithin the Yellowstone Park and just 

 west of the Absarokas occurs a fine example 

 of a volcano, situated near the intersection 

 of the prolongation of the Ishawooa intru- 

 sive body and the fault along the southern 

 slope of the Snowy Eange. Mount Wash- 

 burne is the culminating point of the vol- 

 cano, which consists almost wholly of 

 fragmental early basic breccia. From a 

 well-recognized crater, since partially filled 

 with rhyolite, the erupted material has been 

 thrown out in every direction, building up 

 true volcanic slopes encircling a central dis- 

 charging vent. Such a structure I have 

 never been able to recognize in the Absa- 

 rokas. Mount Sheridan, in the Park, is an- 

 other large volcano, but this is a Pliocene 

 eruption consisting wholly of rhyolite, and 

 is one of the sources of the great body of 

 rhyolite which built up the Park plateau 

 probably long after the Absarokas ceased to 

 be a center of volcanic action. 



Of all known regions of eruptive energy 

 within historical times, Iceland in many 

 ways affords the best field for comparison 

 of the volcanic phenomena of to-day with 

 conditions as they existed during the early 

 Tertiary time in the Absarokas. Iceland 

 is one of the active centers of eruption on 

 a stupendous scale. It offers a continuous 

 volcanic history throughout Pleistocene 

 time, and dates back to the early Miocene, 

 as is indicated by its fossil flora. In early 

 Tertiary time the island was a region of 

 profound faulting, and it is supposed to 

 have been separated from the mainland 

 during that period. Dr. Thoroddsen, the 

 Icelandic geologist, has published in Euro- 

 pean scientific journals most interesting 

 accounts of his explorations over the less- 

 known regions of the island. The most 

 complete and instructive of these accounts 

 which has come to my attention was pub- 

 lished by the Stockholm Society of Science 



in 1888. Notwithstanding the volcanoes of 

 majestic pi-oportions which contribute so 

 much to the scenic grandeur of Iceland, 

 and which must give to all geologists who 

 have seen them a profound sense of the 

 power of volcanic energj'. Dr. Thoroddsen, 

 who has lived among them, protests against 

 the idea that they were built up like Vesu- 

 vius or ^tna. He says: "The vast lava 

 waste of Odadahraun was produced by the 

 eruption of over twenty volcanoes, and per- 

 haps many of the oldest centers of eruption 

 that contributed to the formation of this 

 desert have become obliterated by later 

 lava streams. When one recalls geological 

 text-book descriptions of modern volcanoes 

 and their activity, it is nearly always Vesu- 

 vius that everywhere turns up like a spectre, 

 whereas the regular volcanic cone composed 

 of alternating lavas and tuffs is rather rare 

 in Iceland.'' 



The country which he is describing may 

 be about one-half the size of the Absaroka 

 Eange, but I have no maps or accurate data 

 for determining the ai'ea. Again, later, he 

 says : ' ' Only a few old volcanoes are found 

 having this form. In Iceland it is very gen- 

 erally found that the fissure has not given 

 rise to the formation of any real volcano. 

 The lava there has sometimes welled out 

 along the entire length of the fissui-e with- 

 out the formation of a crater, but mostly 

 there has been formed a series of low slag 

 cones at the points where the magna, by 

 reason of the form of the fissure or for some 

 other cause, found it easiest to break forth. 

 Such rows of craters are found in all vol- 

 canic regions of Iceland." Another notice- 

 able feature, even in the active regions of 

 Iceland, is the ease with which sources of 

 eruption may become obliterated by fresh 

 flows from neighboring vents of discharge. 

 According to Dr. Thoroddsen the famous 

 Heckla itself is a long ridge built up by a 

 chain of small craters along a line of fissure. 



Sir Archibald Geikie, in his admirable 



