308 J. W. JUDD ON THE ANCIENT VOLCANO OF 



any thing like agreement as to the lines of separation between the 

 " syenite " and the " greenstone " — rocks by one author assigned 

 to the former being by others referred to the latter. And it is only 

 necessary for the geologist to study the interesting series of sections 

 exposed in the Eisenbach and Hodritsch valleys, to be convinced 

 that the two classes of rocks pass into one another by the most in- 

 sensible gradations. 



But, on the other hand, it is equally certain, as is, indeed, I 

 believe now universally admitted alike by petrologists aud physical 

 geologists, that the so-called greenstones, greenstone-trachytes, and 

 trachytes are all merely varieties of the same rock, and constitute 

 indeed portions of the same great eruptive masses. 



Now, that the so-called trachytes (or andesites) are true vol- 

 canic rocks, erupted during the Miocene period, we have, as already 

 shown, the most convincing proofs. Hence we are irresistibly led 

 to the conclusion that all these igneous rocks of the Schemnitz area, 

 "granites and syenites," "greenstone-trachytes," andesitic lavas 

 and tuns, are parts of the same great eruptive masses, and are of 

 contemporaneous date. 



The truth of this conclusion is borne out in a very striking manner 

 by a comparison both of the chemical and microscopical characters 

 of the three different classes of rocks in question. In the following 

 table I place side by side analyses of typical examples of the so- 

 called syenite, of the " greenstone-trachyte " and of the andesitic 

 lava of the Schemnitz district. 



I. II. III. 



Silica 61-73 60-26 61-62 



Alumina 17*45 18-25 20-66 



Oxides of iron and 



manganese 5-94 6-83 6-64 



Lime 4-52 3-08 4-27 



Magnesia 2-29 0-77 1-35 



Alkalies 7*00 5-61 4-55 



Loss 1*16 3-40 2-40 



I. is the so-called syenite of Hodritsch, analyzed by Karl von 



Hauer. 



II. the " greenstone-trachyte " of Gelnerowsky Wrch, analyzed 



by Yon Sommaruga. 



III. the hornblende-andesite of Eybnik, also analyzed by Yon 

 Sommaruga. 



Allowing for the different degrees of decomposition, as shown by 

 the amount of volatile material in each case, the rocks are seen to 

 be almost identical in composition. 



When we turn to the consideration of the microscopical characters 

 of the so-called " syenite and granite " of Hodritsch, we find that, 

 while offering the most marked points of contrast with all the un- 

 doubtedly ancient Hungarian granites, it presents some very remark- 

 able features of agreement with the " greenstone-trachytes " and 



