CHEMICAL RELATIONS OE THE VOLCANIC ROCKS. 
I ‘21 
In the Osann formulae, which represent the chemical composition of the rocks, the 
number of molecules of 8i0 2 (including Ti0 2 ) are denoted by s; the alkalies arc united 
to ALO :! (as in the felspars and felspathoids) in a group (NaK) 2 Al 2 0 4 , which is denoted 
by A ; the remainder of the Al 2 O a is united with CaO (as in anorthite) in a group 
CaAl 2 0 4 , which is denoted by C ; and the rest of the CaO with the other metallic 
oxides (FeO, MgO) are united in a group RO represented by F. The number of 
molecules of Na 2 0 is calculated on the assumption that Na 2 O + K 2 O = 10 and is 
denoted by n. Moreover, as the absolute magnitudes A, C, F arc not necessary, since 
2A + 2C + F =100 — .'.', their ratios a, c, /, where a + c + / = 20, are used. The ratios 
a, c, /give an approximate idea of the relative amounts in which the alkali-felspars, 
anorthite and dark constituents, enter into the composition of the rocks. 
For the seven rocks the Osann formulae are as follows under ( I ) (7) 
(1) 
^4804 
Ju\ ^7 35 
I. 
S 00'77 ^2 ^2/10 ^7H 
(2) 
5 B244 
^3J ^/l4 ^703 
II. 
'^U0'78 ^3 CvJ IB ^7'2 
(3) 
,<f <13-1)1 
^11 ^1 fs ^7 23 
nr. 
,<f 03'34 ^10 ^1 ft f hl 
(4) 
•Vi* 72 
«10 ('Vifn J ^7*02 
IV. 
,<? 03'78 f h\ ^4 ./«J 
(6) 
S (H7l 
^J0 ^2i./74 ^7 40 
V. 
5 03'7O ^104 ^7 2 
(6) 
^13j ( 'o /ilj ^0 70 
VI. 
^08'20 ^14 ^0 ./ IS ^7 
(7) 
,S ‘(I7T2 
^10 ^1 fs ^'7 '80 
VII. 
^0714 ^1(1 r oJl ^7 6 
For comparison, opposite the formula of each Antarctic rock, is 
formula (I-Vll) of the following : — - 
T. Nepheline-basanifce from llundskopf, Salzungcn. 
II. Lirnburgite from Heldburg, near Coburg. 
III. Pkonolite from Miidstcin, Bohemia. 
IV. “ Rhomb-porphyry” (kenyte) from Kibo, Kilimandjaro. 
V. “ Haiiyne-tephrite ” from Campanario, Pahna. 
VI. Pbonolite from Bull Cliff, Colorado. 
VII. Pbonolite from Mont Miaune, Velay. 
appended the 
The analytical results show that these Antarctic volcanic rocks do not form 
anything in the nature of a rock-series, but that they may be divided fairly sharply 
into two groups, a basic one consisting of hornblende- and olivine- basalts of limburgite- 
type (1 and 2), and one of medium basicity consisting of kenytes and phonolitic 
trachytes (3-7) very rich in alkalies. 
These two groups appear to represent the two main products into which the 
magma has been differentiated in this Antarctic region. 
A graphical representation (according to Brogger’s method)* of these two dif- 
ferentiation-products, as illustrated by rocks (l) and (5), is shown in Bigs. < >7 and 08 
respectively. 
In an Osann triangle, f the rocks of the first group fall into Division III, and 
those of the second group into the upper part of II, in a similar position to that 
assigned by Becke J to the tephritic and phonolitic rocks of the Bohemian Mittelgcbirge. 
* Briiggor, Eruptivgestcine des Kristianiagebietes, ISOS, J 1 1, p. 2 AO. 
t Osann, Tschermak’s Min. Petr. Mitth. 1900, Bd. xix, p. 868, and PI. IV. 
% Becke, Tsehermak’s Min. Petr. Mitth. 1908, Bd. xxii, p. 214, and PI. II. 
von. i. 
It 
