6 ANALYSES OF ROCKS AND MINERALS, 1880-1908. 
ing constituents to 100 per cent, the comparison of estimates will 
assume the following form: 
Clarke if 
i cee te A . | Washing: 
Tlarker. ton, 
First: Second. | Third 
61. 22 61.12 60. 36 58. 96 
15. 75 15.77 16.07 15.99 
200 2.69 5.48 3.37 
3. 53 3.60 2.46 3.93 
4.51 4. 46 4.20 3.89 
4.93 5.02 5.12 5. 28 
3.69 3.63 3.34 3.96 
2.90 2.87 2.83 3.20 
-61 -12 1.05 
22 23 02 37 
100.00 | 100.00 | 100.00 100. 00 
Between Washington’s computation and mine there is one funda- 
mental difference—a difference of method. In my own calculations 
analyses were averaged together as if each one was complete—an 
assumption which is not justifiable. In some cases minor constituents 
were not reported; in other cases they were determined with great 
care. The average of analyses varying in this way will obviously 
give too low a result for the rarer substances. Washington’s policy 
was to consider the rock-forming elements separately, averaging each 
according to the actual number of determinations made, and thus to 
secure a truer group of estimates. For example, his average repre- 
sents 1,811 determinations of Al,O, and SiO,, 1,625 of Fe,O, and FeO, 
1,139 of TiO,, 955 of P,O,, and only 731 of MnO. The data thus 
utilized, however, coming from many laboratories and representing 
various methods of analysis, are evidently of very unequal value; and 
this consideration counts for something against the result finally 
obtained. I have therefore thought it desirable to repeat the com- 
putation, applying Washington’s method to the determinations made 
in the laboratories of the Survey alone, and using all of the analyses, 
partial or complete, for my purpose. Every determination made 
upon an igneous rock has been thus utilized. When an element was 
definitely reported as absent, it has counted as zero in the averaging. 
To the statement of a “trace” I have arbitrarily assigned the value of 
0.01 per cent. In this way I have obtained the following averages, 
which represent all of the data accumulated by the chemists of the 
Survey down to January 1, 1908: 
Silica.—Average of 1,530 determinations, 60.97 per cent. 
Alumina.—Average of 1,042 determinations, 15.29 per cent. In 93 additional deter- 
minations, TiO, and P,O,; were not separated; and these, therefore, are not in. 
cluded in the computation. 
Ferric oxide.—1,093 determinations, Average, 2,63 per cent, 
