Geological Society of London. 279 



Besides the rhyolites and basalts, however, there are certain 

 other rooks exposed in the central area of the Schemnitz district, 

 the relations of which it is very difficult to understand. These 

 consist of (1) strata of Lower Trias and Nummulitic age, through 

 the midst of which the volcanic outbursts have evidently taken 

 place ; (2) masses of highly metamorphic rocks, including quartzites, 

 crystalline limestones, various schists, gneiss and aplite ; and (3) 

 undoubted eruptive rocks, which have usually been called " syenite 

 and granite," but for which the names of " diorite and quartz- 

 diorite " would perhaps be more appropriate, inasmuch as the pre- 

 vailing felspar in them is always a plagioclase varietj-. 



By Beudant and other early writers the andesitic lavas were 

 recognized as volcanic products of a comparatively recent geological 

 period, while the ''granite, syenite, and greenstone" were regarded 

 as being of far more ancient date. By Von Pettko, Eichthofen, and 

 all the more recent investigators of the district, however, it has been 

 clearly perceived that the " greenstones " are certainly, like the 

 andesites, of Tertiary age, and hence such names as " greenstone- 

 trachyte " and " propylite " have been applied to them. The 

 studies of the author of the present memoir, both in the field and 

 in the cabinet, have led him to the conclusion that the granitic, 

 porphyritic, and lava rocks — which were formerly called " syenite," 

 "greenstone," and "trachyte" respectively— are all of similar com- 

 position and equivalent age, and that they differ only in their more 

 or less perfect state of crystallization, the result evidently of varia- 

 tions in the conditions under which they have consolidated. He is 

 farther led to regard the metamorphic masses around the several 

 intrusive centres as being not, as has hitherto been maintained, of 

 a Primary " (Devonian or Permian) age, but simply Triassic rocks 

 afi'ected by local or contact metamorphism. 



The real structure of the great Schemnitz volcano was first recog- 

 nized by Yon Pettko in 1848, though this author erroneously 

 regarded it as presenting an example of a " crater of elevation." 

 The history of the formation and destruction of this volcano is now 

 shown to be as follows : — After some small and scattered outbursts 

 of rocks of acid composition towards the close of the Oligocene 

 period, the grand eruptions of andesitic lavas of the Miocene began, 

 through the agency of which a volcano of larger dimensions than 

 Etna was gradually built up, by both central and lateral eruptions. 

 In the midst of this volcano a crater of enormous dimensions was 

 formed, doubtless by some great paroxysmal outbreak, and by the 

 subsequent subsidence of the mountain the sea gained access to, and 

 by denudation greatly enlarged the area of this " Caldera." Then 

 in the central lagoon of the caldera a number of minor eruptions, 

 first of acid and then of basic rocks, took place ; and the volcano, 

 which at this period of its history must have closely resembled the 

 existing island of Santorin, was again upheaved from beneath the 

 sea, and exposed to the wasting effects of subaerial denudation. 

 The gradual decline of the volcanic forces in the district was marked, 

 as is usually the case, by the appearance of hot and mineral springs, 

 discharges of gas, occasional earthquakes, etc. 



