(') ox HAUYNE-TRACHYTE AND ALLIED ROCKS. 



increases in quantity, and ^^•o^vs more basic, ^ve are led to 

 the andesites ; and it is not always eas}' to understand the 

 meaninjo: of the term trachyte as used by some authors, 

 who extend it in the direction of the andesites till it 

 becomes useless for the purpose of classification. In this 

 connection "we cannot do better than bear in mind Rosen- 

 busch"s definition of trachyte as implying the dominant 

 presence of a potash felspar and the absence of quartz 

 among the porphj'ritic constituents. He says : — 



" The trachytes are neo-volcanic effusive rocks ^vhicll 

 may be designated equivalents of the syenitic plutonic 

 rocks and of the palaeo-volcanic quartzless porphyries in 

 all their modifications."* 



The only modification which \se would venture to make 

 in this admirable description would be to include palaeo- 

 volcanic rocks in the trachyte grouj). Some carboniferous 

 trachytes have been found in Britain, but these have Ijeen 

 ranked by the illustrious author just named anaong 

 " orthophyres " or " quartzless porphyries having com- 

 pletely the habit of trachytes." We cannot see, apart 

 from the question of age, that anything would be gained 

 by calling the Port Cygnet lavas, orthophyres. Many of 

 the felspars are brilliant to the eye and pellucid in thin 

 section. On the other hand, some of them have lost their 

 glassy appearance and acquired a yellowish opaque aspect, 

 sometimes, however, retaining a vitreous interior, to which 

 the peripheral decomposition has not extended. We seem, 

 here to have intermediate stages between glassy sanidine 

 and opaque orthoclase. No doubt, the mineralogist would 

 deny the term sa"nidine to these opaque crystals and call 

 them orthoclase. Of course, those who postulate sanidine 

 as an essential constituent of trachyte will have to call 

 some of these rocks trachytic orthophyres, and reserve 

 trachyte as the name for the varieties with glassy ortho- 

 clase. But this seems to us a needless multiplication of 

 classes, and w^e anticipate that the sanidine variety of 

 orthoclase will not always be regarded as an essential 

 ingredient of trachyte, nor will the name trachyte be 

 confined to rocks of Tertiary age solely. 



Assuming, then, that we are now dealing with the 

 trachytic group, we take a further step and define these- 

 volcanic rocks as soda trachytes. This is shown hj the 

 presence of one or more of the soda minerals, haltyne,. 

 analcime, aegirine, aegirine-augite, cataphoritic hornblende, 

 and the green pleochroic augite rich in the acmite 

 molecule (Xa Fe Sig Og). Such trachytes are very closely 



* Mik. Phys, d. massigen Gesteine, 189G. p. 738. 



