COMPOSITION OF ROCKS. 5 
line rocks. Of these analyses only 207 were from the laboratories of 
the Survey, while 673 were collected from various other American and 
foreign sources. A large proportion of them were incomplete, re- 
garded from a modern point of view, and yet the results obtained 
were fairly conclusive. 
In Bulletin 148 a similar estimate was given, based upon 680 
complete analyses found in the Survey records, plus some hundreds 
of determinations of silica, lime, and alkalies. Again, in Bulletin 168, 
a third estimate was presented, representing 830 complete analyses 
and some partial determinations, all made in the Survey laboratories. 
In 1899 Harker published a computation covering 397 analyses of 
British rocks, and Washington? has also worked out the average of 
1,811 analyses given in his compilation. 
For the more important constituents of igneous rocks the five esti- 
mates mentioned above agree remarkably well, and yet they are not 
thoroughly comparable. The 397 analyses discussed by Harker were 
in most cases incomplete, at least when considered from a modern 
standpoint. In only 34 of them was titanium taken into account, and 
in only 55 is phosphorus mentioned. These omissions affect the per- 
centages of other things and lessen the value of the computation very 
materially. Excluding manganese the five averages may be tabulated 
as follows. Minor constituents will be considered later: 
Clarke 
= Harker. Wasting: 
First. | Second. | Third 
isifey eRe ane vere eRe ITS RO eae TEER 58.59| 59.77) 59.71| 58.75 58. 239 
WOW. oye ae tec desee ecces ee 15.04| 15.38) 15.41| 15.64 15.796 
Fes Ossian hae ore eas eee eta eee 3,94 2.65 2.63 5.34 3,334 
FeO 3,48 3.44 3,52 2. 40 3,874 
MeO ee ee eaten 4.49 4.40 4.36 4.09 3. 843 
CaO oe kee orccessece ee seecegaee eceeme st 5.29 4.81 4.90 4.98 5.221 
INanO sence ers be ass acen ray Sine 3,20 3.61 3.55 3.25 3.912 
KG sen coh Geese ieee bee a sep “] 290 2.83 2,80 2.74 3.161 
E30 Ab 1008 ce cree case: ae ona ces 5 my cota las ent beesseesee : 
Hy0 above 100°22.222222222IIE “Tp 198 isi 1152 } 2.23 1,428 
MI Oaaeht ese ge eee 35 153 60 12 1.039 
PHO eos Soe en oe cade eel eee 22 121 122 102 373 
99.66] 99.14] 99.22] 99.56 100. 583 
An examination of the foregoing table will show several discrep- 
ancies, and one of them is in the variable treatment given to water. 
In two columns hygroscopic water does not appear; in two others total 
water is given; in Washington’s estimate a discrimination has been 
made. If we reject the figures for water and recalculate the remain- 
a Geol. Mag., 4th decade, vol. 6, p. 220. 
b Prof. Paper U.S. Geol. Survey No. 14, 1903, p. 106, 
15619—Bull. 419—10 2 
