RECENT WORK ON VOLCANOES 93 



for the ordinary strato-volcanoes which are built up of successive 

 layers of ash and lava-flows from the actual chimney that they 

 surround ; in the Hrossaborg the lava and ashes are older, and 

 came from other volcanic vents or fissures. 



From these explosion rings, or Gasmaare, as Beck calls 

 them, we pass to the well-known crater rings surrounded with 

 low crater walls formed of tuff and lava ejected from the volcano. 

 Many examples occur in Iceland and Idaho, but no special 

 mention of these is necessary here, unless to point out that 

 in Idaho vast streams of lava issued from them. These heads 

 of the lava columns are covered with scoriaceous and ropy lava, 

 which makes them look like the tops of great springs of water 

 suddenly congealed. In one case, a particular lava-flow had its 

 origin in two such pools, and a mile from its source it was 

 joined by a still larger river of lava. The united streams flowed 

 some thirty miles, descending about a thousand feet, more than 

 half of the fall being in the first ten miles, so that the distal 

 portions flowed on a gradient of i in 200. Other streams have 

 flowed for fifty miles in the same area in rivers of molten rock 

 one to three miles across and 300 ft. in thickness. 



Slag Craters. — Two volcanoes of this type are described 

 by Russell from Idaho. Blanche Crater rises about 60 ft. 

 above the plains, and has a perfect crater on top ; the conical 

 pile is composed of thin cakes of highly vesicular lava, which 

 have been blown out in a plastic or liquid condition. It is of 

 quite recent origin, as it lies in a canyon excavated 500 ft. in 

 the older lava. The other example is the Martin Butte, like- 

 wise a conical pile of scoriaceous lava. In Iceland, slag cones 

 are extremely common and form the most weird objects in the 

 landscape, as the viscid lava has built up piles of all shapes, 

 resembling towers, organ-pipes, needles, or gigantic skittles (12). 

 They vary from 150 ft. in height to quite small hornitos or 

 blowing and driblet cones. They are often assembled in 

 swarms, as if a great mass of gas had pierced a viscid covering 

 along a number of independent channels. They frequently 

 form, also, the caps of the next type of volcano. 



Buckler Cones. — One example has been described from 

 Idaho, the Black Butte; it rises 300 ft, with a base two miles 

 in diameter. It is built up of successive layers of highly 

 scoriaceous lava, which flowed away in all directions, and there 

 is no evidence at all of lapilli or cinders. There is no crater 



