318 



J. W. JUDD ON THE ANCIENT VOLCANO OF 



rocks are never seen resting directly upon a denuded surface of the 

 granite*, but are always separated from it by a zone of highly meta- 

 morphosed rocks. 



The most satisfactory method of determining the true relations of 

 the igneous, metamorphic, and sedimentary rocks of this district is 

 by studying their exposures along the Hodritsch, Eisenbach, and 

 Glashiitte valleys and in the mouutain-masses of the Kohl-Berg 

 (see fig. 1) and the Schwatzer Berg (see fig. 2) which separate 



Fig. 1. — Section through the Kohl-Berg. 



Schuttris- 

 8. berg. 



Glashiitte 



valley. N. 



Werfener 

 Schichten 



Limeston 

 dolomite 



3 



sn. I H 



e and f $ 



p. Schistose rocks. ^ g> 



q. Quartzite and I 2 



Quartz- Schiefer. | a§ 



r. Aplite. J. o'C 



s. Gneiss. I S^ 



t. Crystalline | -g 



limestone. ) g 



Syenite" (dio- 

 rite). 



Greenstone - tra- 

 chyte " (Horn- 

 blende - ande- 

 site) . 



these valleys. Here we find, in innumerable instances, the lime- 

 stones, sandstones, and shales of the Trias, wherever we approach 

 a mass of " syenite "or " greenstone- trachyte," undergo a marked 

 change and acquire by the most imperceptible gradations a more 

 crystalline character, till finally they assume the condition of a 

 crystalline or schistose limestone, of a quartzite or quartz-schist, or 

 even of a gneiss or aplite (granulite). I believe that it is impos- 

 sible for a geologist to examine all the interesting lines of junction 

 between the igneous and stratified rocks without being fully con- 

 vinced that the metamorphic masses which in every instance occur 

 between them have been produced by the action of the Miocene 

 intrusive rocks upon the Triassic strata which they have penetrated 

 (see fig. 3). 



I do not propose in this place to enter upon a discussion of the 

 details of the interesting problems which are so admirably illustrated 

 by these beautiful examples of local or contact metamorphism, and of 

 their bearing on the wider question of the nature and causes of the 

 phenomena of genercd metamorphism. On a future occasion I hope 

 to lay before this Society the results at which I have arrived from 

 the study and comparison, both in the field and in the cabinet, of 

 the rocks which exhibit the most distinct and rapid transition from 



* In this place, as in many other portions of this memoir, I have used the 

 names by which the rocks of the district have been so long indicated, rather 

 than those which I have endeavoured to show in the present paper would be 

 more appropriately assigned to them, in order to make my line of reasoning 

 more clearly manifest. 



