216 ALEXANDER N. WINCHELL 
is black and dense; but its separation from the andesite is accom- 
plished at a glance when both rocks are fresh. 
A substitute for the compound name olivine gabbro is desirable 
but is not here suggested. 
The average composition of each type of igneous rock should 
be available for comparison with the composition of rocks at given 
localities. Dalyt has prepared tables giving these averages for cer- 
tain rock types; in the following tables these have been supple- 
mented by averages computed by the author. 
In the tables of average composition, numbers I, 4-7, I0, 13, 
NO, EQ, 21. 22) 25. 20, 20-345 20, 28)410,140-52.154=517) Wwiehercd eulamed: 
by Daly;? the others have been prepared from data published by 
Rosenbusch (Elemente der Gesteinslehre, 3d edition, Stuttgart, 1910), 
and by Clarke (U.S.G.S. Bulletin 419, 1910), with the following 
exceptions. No. 3 is based on data of Rosenbusch (loc. cit.), Clarke 
(Joc. cit.), and Osann (Chemische Petrographie, 1, 1905). No. 9 is 
from data of Rosenbusch (loc. cit.), Clarke (Joc. cit.), Osann (loc. cit.), 
and Weidmann (Wis. Surv. Bull. XVI, 1907). No. 14 is based on 
data of Rosenbusch (Joc. cit.), Washington (Jour. Geol., VII [1899], 
57), and Ogilvie (Jour. Geol., XVI [1908], 285). No. 15 is based on 
data of Rosenbusch (loc. cit.) and Osann (loc. cit.). No. 17 is from 
data of Rosenbusch (Joc. cit.), Lacroix (‘‘Minéralogie de Madagas- 
car,’ Nouv. Arch. Muséum, I [1903], 30, 194), and Young (Geol. 
Surv. Canada, XVI, H, 1906). No. 27 is from data of Rosenbusch 
(loc. cit.) and Hore (Econ. Geol., VI [1911], 54). No. 29 is from data 
of Rosenbusch (loc. cit.) and Doelter (Akad. Wiss. Wien, CXI, I 
[1902], 980). No. 37 includes 12 quartz keratophyres (Daly), 13 
pantellerites (data of Rosenbusch), and 6 comendites (data of 
Rosenbusch). No. 44 is from data of Daly (loc. cit.), Clarke (loc. 
cit.), Rosenbusch (Joc. cit.), and Osann (Joc. cit.), auganites excluded. 
No. 47 is from data of Clarke (Joc. cit.), Rosenbusch (loc. cit.), and 
Osann (loc. cit.), rocks containing acid plagioclase or orthoclase 
being excluded. 
tR. A. Daly, Proc. Amer. Acad., XLV (1910), 211. 
2 Loc. cit. 
