92 SCIENCE PROGRESS 



Iceland is an elevated portion of a plateau of basalt and 

 pelagonite tuff that at one time stretched in a continuous field 

 from Antrim in the north of Ireland to Greenland, a distance of 

 a thousand miles. The Faroe Islands are an isolated remnant 

 of this plateau ; all the rest has sunk beneath the sea. The first 

 of the lavas rests on the topmost beds of the Cretaceous system 

 in Scotland, so that presumably the eruptions commenced in 

 Eocene times and are still going on at the present day in the 

 northern part of the area. 



On the other side of the Atlantic, in Oregon, Washington, 

 California, Idaho, and Montana, an extent of country larger 

 than France and Great Britain combined has been flooded with 

 basalt ; the previous topography has been buried under lava 

 2,000 ft. thick and in some places 3,700 ft. thick, the surface of 

 which is a level plain like that of a lake-bottom. In the Snake 

 River Plains, a part of the larger area, the lava rolls up to the 

 base of the hills on the north and on the east and follows the 

 sinuosities of their margin as the waters of a lake follow its 

 promontories and bays. The basalt rests on beds of lapilli 

 which may reach 180 ft. in thickness, and these in turn rest on 

 lacustrine deposits. I follow I. C. Russell's description of 

 this area (11). 



Explosion Rings. — These are the more primitive forms of 

 what Judd calls crater rings, of which many examples occur in 

 Italy, such as the hollows in which lie the lakes of Bolseno, 

 Bracciano, Albano, Nemi, and Frascati. The simplest of all 

 occur in Idaho near Cleft, where there are two circular holes 

 drilled through the basalt without any elevated rings. Their 

 diameters are 1,100 and 800 ft. ; the encircling cliffs rise 200 ft. 

 above the floor, which is composed of fine yellow soil. In 

 Iceland (13) we find a slight development; the type is the 

 Hrossaborg, near Akureyri, the capital of North Iceland. Here 

 the plains consist of doleritic lava overlying pelagonite tuff, and 

 the volcanic eruption has lifted up a portion in the form of 

 a circular hill with a crater, some 800 yards in diameter, on top. 

 The only products of the volcano were gases which have drilled 

 the circular chimney and elevated the rocks around. The inner 

 walls of the crater are 120 ft. high, and on all sides the rocks 

 slope outwards. It is a typical crater of elevation according to 

 Leopold von Buch, only unfortunately we cannot apply this 

 term to this type now, as the original name was used erroneously 



