THE DISTRICT OF SCHEMNITZ, HUNGARY. 



303 



thus given rise to the formation of a rather extensive lake, which 

 has been partially filled up with sediments and then drained by the 

 cutting-through by the stream of the barrier of lava which retained 

 it. This comparatively modern stream of lava, however, is of ande- 

 sitic composition; and the cone is formed by scoriae of the same 

 rock. 



The following table exhibits the average composition of the most 

 characteristic varieties of volcanic rock which have been erupted in 

 Hungary. These averages have been calculated from a considerable 

 number of published analyses. 



I.Ehyolites II Dacites g^te^ IV . Augite . 

 or Quartz- or Quartz- Mi _„_ Ande . Andesites 

 Trachytes. Andesites. sit ^ Andesites. 



Specific gravity .. 2-375 2-629 2-643 2-715 



Silica 73-49 65-03 58 49 56-21 



Alumina 13-63 14-80 20-45 17-26 



°*;r d i »* •* ^5 n-56 



Lime 1-24 3-61 5-58 6-79 



Magnesia 0-32 1-01 1-72 1-49 



Alkalies 7-23 5-58 6-19 4-43 



The quantities of soda and potash are not given separately in these 

 averages; for in the analyses of Yon Sommaruga, on which they are 

 based, the separation of the alkalies was effected by a method which, 

 as has been since shown by Tschermak and Doelter, does not yield 

 reliable results. 



The volcanic formations of Hungary offer some peculiarities of 

 great interest, especially in the remarkable abundance of rocks 

 mainly composed of plagioclase felspar, with or without free quartz. 

 To the British geologist they are of especial interest, on account of 

 their numerous and striking points of analogy with the newer Pa- 

 laeozoic volcanic rocks of Scotland. Indeed we may safely assert 

 that the Palaeozoic hornblende-, mica-, and augite-porphy rites of our 

 own island present the exact counterparts of the Tertiary hornblende-, 

 mica-, and augite-andesites of Hungary ; and, excepting in the condi- 

 tion of their preservation, I know of no grounds whatever for placing 

 them in different categories, and assigning to them distinct names. 

 Rocks of precisely similar composition to those of Hungary are 

 abundantly developed in various parts of the Turkish Empire ; and 

 in the island of Santorin, as we have already remarked, they are 

 being erupted at the present day. 



No contrast, however, can possibly be greater than that afforded 

 by the volcanic rocks of Hungary on the one hand, and those contem- 

 poraneously formed in Bohemia and Central Germany on the other. 

 "While, in the former, plagioclase felspars almost always predominate, 

 even when the rocks are highly quartziferous, in the latter ortho- 

 clase is everywhere the most abundant constituent, occurring in 

 rocks even of the most basic type. On the north-west side of the 



