Classification of the Eruptive Rocks. 375 



I have always noticed that the micaceous labrador-trachytes occur 

 in the breccias containing fossils of the Middle Miocene (Mediter- 

 ranean stage) ; the oligoclase-trachytes and the orthose-trachytes 

 are also met with sometimes on this horizon, but never those of the 

 anorthite-pyroxene type. This indicates that the latter types of 

 trachj'te are of later date. The trachytic tuff containing fossils of 

 the Upper Miocene (etage Sarmatien) consists chiefly of anorthite- 

 trachytes, but trachytes of the other types may also be recognized 

 in it, and we may therefore conclude that this anorthite-trachyte is 

 more recent than that containing labradorite. In order to learn the 

 age of the oligoclase-trachytes and orthose-trachytes I have examined 

 the oldest tuffs ; and this question is now limited by reason of the 

 discovery of trachytic fragments in the Upper Eocene deposits of 

 Budapest, associated with the Nummulites and the other Foraminifera 

 characteristic of this formation (etage) ; these trachytic fragments 

 are orthose-trachyte. In the district of Gran, in a higher horizon, 

 which, however, still belongs to the Upper Eocene, one meets with 

 sheets of oligoclase-trachyte, two or three metres in thickness, 

 without the least trace of lab rador- trachyte or of anorthite-trachyte. 



The orthose-trachytes and the oligoclase-trachytes are, therefore, 

 the oldest rocks of this eruptive series, while the anorthite-trach3'te 

 is demonstrably the most recent. As an additional proof of this 

 it may be stated that the anorthite-trachyte traverses all the other 

 types, but is itself cut by none of them. 



These observations lead to the following important conclusions : — 



1. Contact phenomena exist, and at the limit of two different 

 types, there is sometimes a mixture of the minerals belonging to 

 the two adjacent types. 



2. The different types have undergone important modifications, 

 which deserve special designations. It, therefore, becomes necessary 

 to recognise a normal and a modified condition for each trachytic 

 type ; while the older the type, the more it is modified. The 

 anorthite-trachyte is the one which most frequently occurs in a 

 normal condition. 



The principal modifications are known by the following names : 

 rhyolite, litlioidite, trachytic-greenstone, domite, millstone-porphyry, 

 alunite. 



Bhyolite, accepting the term strictly in the sense in which it has 

 been proposed by Eichthofen, comprises obsidian, perlite, pitchstone, 

 and pumice ; it is formed last, during the eruption of one of the 

 most recent types by the intervention of very fusible hydrated 

 silicates, and it is these hydrosilicates which for the most part give 

 rise to fluxion structure. 



The presence of the rhyolitic modification is then an important 

 character for determining relative age, for it always leaves the 

 supposition of the existence of a more basic trachyte. 



Of all the types the orthose-trachyte affords the most perfect 

 rhyolites, the oligoclase-trachyte much less so, while the labrador- 

 trachyte never forms either obsidian or pitchstone, but it becomes 

 feebly perlitic and pumiceous. The anorthite-trachyte becomes merely 



