THE VOLCANIC ROCKS OF ICElLAND. 35 



The rocks of Iceland, and probably those of most other vol- 

 canic systems, may be classified into two principal groups, 

 which, notwithstanding the gradual transition by which they 

 are united, may still be easily recognized in their extreme mem- 

 bers, and distinguished as normal-trachytic on the one hand, 

 and as normal-pyroxenic on the other. The individual members, 

 frequently presenting great petrographic differences, maintain 

 in each of these groups a peculiar, but perfectly constant com- 

 position*. 



The first distinguishing characteristic of the normal trachytic 

 rocks is, that they represent almost exactly a mixture of bisili- 

 cates of alumina and alkali, in which lime, magnesia, and prot- 

 oxide of iron are either wholly wanting or present only in in- 

 significant quantities. The following analyses show the com- 

 position of the principal members of this group. The variety of 

 rock 1 constitutes the chief mass of the trachytic cone of Baulaf. 

 Its colour is white, frequently with a yellow or bluish tinge ; it 

 is rough, moderately granular, and has a great number of small 

 irregular cavities. 



The variety 2, from Stnitrhals, near Kalmanstiinga, four 

 Danish miles south-east of Baula, forms a perpendicular, mass- 

 ive, irregularly-fissured precipice, whose base is washed by the 

 Nordhlingafljot. This rock is traversed by a trap-dyke. It is 

 of a brilliant white, has a distinctly crystalline granular struc- 

 ture, and in other respects resembles that previously mentioned. 

 The variety 3, from Laugarfjall, near the Great Geyser, is in 

 large columns; it forms a compact, bluish-gray mass, large 

 fragments of which present an indistinct angular fracture, and 

 small fragments a splintery fracture with isolated felspathic con- 

 cretions, but imperfectly distinguishable from the matrix, and 

 an extremely small number of minute crystals of hornblende. 

 The variety 4 is from a massive trachyte, occurring in the form 

 of crags in the Arnarhnipa (Eagle's Rock), on the banks of the 

 Laxd, between Hruni and Storinupr. It is not columnar, and 

 resembles the rock of Strutrhjlls, but is yellower, has a finer 



• I understand by the former the trachytic rocks richest in silica, and by the 

 latter the basaltic and doloritic rocks with the smallest quantity of silica. 



t Forchhammer considers this rock to consist of a hitherto unknown variety 

 of felspar, which he calls Baulite. 



D2 



