G. W. Tyrrell—Petrographic Nomenclature. 497 
olivine-basalts ; when free from olivine they have been designated 
either as basalt, felspar-basalt (Fr. labradorite), or, as frequently, 
andesite. 
The reasonableness of Iddings’ position is demonstrated by a study 
of the quantitative mineral composition of average andesite and 
average basalt. Professor R. A. Daly has computed the average 
andesite and basalt from the analyses of rocks named as such in 
Osann’s lists.1 These averages are set out for comparison in 
Table I, 1, 2. 
TABLE I, 
CHEMICAL COMPOSITION OF AVERAGE ANDESITE AND BASALTS. 
qe D. Bh 
SiO, 59°59 49-06 47°20 
TiO, a7] 1:36 2:44 
Al,Os 17°31 15:70 14:93 
Fe.03 3°33 5°38 4:11 
FeO 3°13 6:37 8:23 
MnO 18 31 25 
MgO 2°75 6°17 6°92 
CaO 5°80 8°95 10:02 
Na,O 3°58 3:11 Oral 
K,O 2-04 ES, 84. 
H,O 1:26 1-62 ANS 
P.O; 26 "45 33 
| 
100-00 100-00 100-11 
1.-—Daly’s average andesite, tonalose, 'II,4.'3.4. Igneous Rocks and their 
Origin, 1914, p. 26. Anal. No. 46. 
2.—Daly’s average basalt, andose-camptonose, (11) I11.5.3.'4. Op. cit., pv. 27, 
Anal. No. 53. : 
3.—Average plateau-basalt, awvergnose, III.5.(3) 4.4’. Computed from thirty- 
seven analyses of plateau-basalts from the Brito-Arctic region (Siberia, 
North Russia, Franz Josef Land, Spitsbergen, Greenland, Jan Mayen, 
Iceland, Faroe Islands, and the Inner Hebrides); the Columbia and 
Snake River plains, Western United States; and the Carboniferous 
Clyde plateau, Midland Valley of Scotland. Analyses obtained from 
- H. 8. Washington’s “ Analyses of Igneous Rocks, 1883-1913’, Prof. 
Paper 99, U.S.G.S., 1917; A. Holmes, “ The Basaltic Rocks of the 
Arctic Region’’, Mineral. Mag., xvii, 1918, 180-223; and H. G. 
Backlund, “‘ Qn the Eastern Part of the Arctic Basalt Plateau,’ Acta 
Academiae Aboensis, Math. et Phis., i, 1920, 1-53. 
These computations are not ideal, for there can hardly be any 
doubt but that each of them covers too wide a range of rocks. The 
andesite average contains rocks which to-day would be designated 
_as dacite, latite, perhaps even trachyte. The basalt average no 
doubt contains rocks which would nowadays be called andesite ;_ but 
this, perhaps, is partly compensated for by the inclusion of rocks 
such as limburgite, trachydolerite, basanite, etc., which diverge in 
various directions from true basalt. Daly’s average andesite is thus 
probably more silicic and alkalic than the real average andesite ; 
while the average basalt is probably defective in the same direction, 
although to a less extent than the andesite. However, these 
1 Tgneous Rocks and their Origin, 1914, pp. 26-7. 
VOL. LVIII.—NO. XI. 32 
