Huttron.—On the Thames Gold Fields. 281 
A B Cc D E F 
Max. Min. Mean 
Silica ...| 64 50: 56:5 | 68°38 | 66°39 62:83 57:17 | 53:85 
Alumina ... | 24° 13: 17:86 | 13:92 | 17°74 | 21°25 | 16:9 179% 
Oxide of iron ... 13: 2- 910, 2:8 | 4:97| 411) 855 | 694 
Lime ж IO O esis. MI bet 7721 63 | 833 
Magnesia... Pie 45 6 | 149) 220] 47| 42| 19 | 647 
Alkalies ... zb: 24 | 376| 753| 4:99| 637| 49 | 3325 
Water... | 7°(2)| 3:44| 563| 4°64] 4:89] 4-15 | 3:38) 2°55 
Specific gravity ...| 2:68 | 229| 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). 
e same (Bischof). 
E Trachyte from Gleichenberg, Styria. Resembles felstone porphyry, compact, 
and of a greyish-green colour with a few felspar crystals (Bischof). 
F Trachyte. Hrad Mountain, Hungary. Matrix fine-grained, grey, rather porous, 
and very hard. There are a few very small lamin of felspar and hornblende in it 
(Bischof). - 
The chemical composition of these rocks, it will be noticed, is similar; E 
especially is remarkably 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 
Mr. 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 ordinary sense of the term, but appears to have 
welled up in a manner different from anything that has been observed on Ше 
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 
