498 G. W. Tyrrell—Petrographic Nomenclature. 
analyses, when calculated into the norms, which closely correspond 
to the modes in essentials, illustrate well the fundamental] differences 
of andesite and basalt. Andesite is a dofelsic type, basalt mafelsic, 
more than five-eighths of the total mineral content in andesite 
belonging to the felsic group of minerals, and less than five-eighths 
in basalts. This fact is illustrated by the norms (tabulated in 
Table II), the average andesite falling into tonalose, 11.4.3.4. and 
the average basalt into camptonose II1.5.3.4. : 
TABLE II. 
Norms of AVERAGE ANDESITE AND BaSatt. 
Ile O, By 
Quartz H 14:70 _- — 
S ee ; 11°68 8°90 5:00 
| Albite. : 30°39 26:20 23:06 
Anorthite . 25°31 24°46 25°85 
Diopside . 1-77 13°87 17°70 
[yperstienc 8-04 11°86 10°17 
Olivine ; -— 1:87 4°85 
~ | Magnetite . 4°87 7°89 6-03 
Ilmenite . Tea 2-58 4°56 
Apatite : ‘ol 1:01 67 
Ratio S/F 4:89 1°53 1222 
The norms are arranged in the same order as the analyses 
in Table I. 
No petrographer has any hesitation in classing the dense, heavy, 
generally olivine-bearing rocks of the great basalt floods (Brito- 
Arctic region, Deccan, Western America, etc.) with the true 
basalts ; the types about which there is doubt as to their position, 
labradorite-andesite, bandaite, andesitic basalt, basaltic andesite, 
olivine-andesite, olivine-free basalt, felspar-basalt, and the like, 
are mostly to be found in andesitic provinces. Daly’s average 
basalt includes rocks of both types, and is therefore both more 
felsic, and the felspar more alkalic, than in true basalts. An 
average computed from the analyses of basalts belonging to the 
plateau-basalt regions of the world should be distinctly more 
basic than Daly’s average basalt. This was found to be the case 
on computing the average of thirty-seven plateau-basalts (Table I, 3) 
from the Brito-Arctic region, the Columbia and Snake River plains of 
Western America, and the Carboniferous Clyde plateau of the Mid- 
land Valley of Scotland. This average falls into the subrang 
auvergnose, III, 5.4.4-5, and its norm is stated for comparison in 
Table II, 3. 
The general increase in basicity (maficity) from andesite to 
plateau-basalt is well shown by the diminution of the S/F ratio. 
The border-line between andesite and basalt should be at the ratio 
S/F 1:67, if the distinction be made on the basis of the ratio of 
felsic to mafic constituents. In this connexion it should be pointed 
out that the norm ratio is always higher than the mode ratio, 
since the method of computing the norm always ensures the 
