HuTTON. — On the Tlmmes Gold Fields. 



281 





A 



B 



O 



D 



E 



F 



Max. 



Mill. 



Mean 













Silica 



64- 



so- 



56-5 



68-38 



66-39 



62 83 



57-17 



53-85 



Alumina ... 



24- 



ls- 



17-86 



13-92 



17-74 



21-25 



16-9 



17-95 



Oxide of iron 



13- 



2- 



910 



2-8 



4-97 



411 



8-5 



6-94 



Lime 



10- 



•6 



3-77 



•84 



•53 



•72 



6-3 



8-33 



Magnesia 



3-6 



•6 



1-49 



2-20 



■47 



•42 



1-9 



6-47 



Alkalies ... 



6- 



2-4 



3-76 



7-53 



4-99 



6-37 



4-9 



3-25 



Water 



7-(?) 



3-44 



5-63 



4-64 



4-89 



4-15 



3-38 



2-55 



Sj)ecific gravity ... 



2-68 



2-29 



2-45 



2-57 







2-64 





A Bed-rock of auriferous veins at the Thames and Coromandel. The mean is from 

 six analyses, which are all that are published. 



B Trachyte porphyry lava from Monte Guardia, Lipari. Resembles a compact 

 clay-stone, and often contains imbedded fragments of augite rock (Bischof). 



C Trachytic conglomerate of the Ofenkuhlen, Homogeneous, white, and thinly 

 stratified (Bischof). 



D The same (Bischof) . 



E Trachyte from Gleichenberg, Styria. Resembles felstone porphyry, compact, 

 and of a greyish-green colour with a few felspar crystals (Bischof). 



P Trachyte. Hrad Mountain, Hungary. Matrix fine-graiaed, grey, rather porous, 

 and very hard. There are a few very small laminse of felspar and hornblende in it 

 (Bischof). 



The chemical coraposition of these rocks, it will be noticed, is similar ; E 

 especially is remai-kably like the Thames rock, both chemically and physically. 

 They are all called trachytes, and are all characterized by containing a large 

 amount of water of constitution ; at the same time they differ among one 

 another quite as much as do the most different varieties of the tuff rocks from 

 the Thames that have, as yet, been analyzed. Of course the dyke rocks must 

 not be compared with these ; they are more basic, and do not contain 

 auriferous veins at the Thames. Hitherto I have called the bed-rock at the 

 Thames a tuff, or tufa, and in this I have been followed by Dr. Hector and 

 Ml*. Davis; but a recent examination of the Malvern Hills has led me to 

 doubt the propriety of the name. This rock has undoubtedly not been 

 ejected in the fragmental state that is implied by the word " tufa," neither has 

 it flowed over as a lava in the oi-dinary sense of the term, but appears to have 

 welled up in a manner different from anything that has been observed on the 

 surface of the earth. On the whole I believe it to be more nearly allied to a 

 lava than to a tuff, and I consequently prefer the word trachyte to that of 

 trachyte tufa. 



Tl 



