84 PETROLOGICAL ABSTRACTS AND REVIEWS 
carbonate rocks, then in terms of minerals, and finally in terms of 
chemical constituents. 
For example, it is determined that the average analyses of shale, 
sandstone, and limestone should be combined in the proportions of 
81.7, 12.05, and 6.25, or in round numbers 82, 12, and 6, to give an 
analysis as nearly as possible like that of the average igneous rock. If 
the average igneous rock is represented by some combination of granite 
and basalt, these being the most widespread types of igneous rocks, it 
should be possible, say the authors, to determine what proportions of 
granite and basalt will give an average that could be approximated by | 
some combination of shale, sandstone, and limestone. For their granite 
and basalt analyses they use the average values computed by Daly. 
But this brings in a loose use of the terms granite and basalt, due to the 
fact that Daly’s averages were computed from analyses of rocks which 
were not in every case actually granites and basalts but which had simply 
been given these names (in many cases certainly not in their modern. 
sense) by the various geologists describing them. Instead of using these 
average analyses, it would have been much better if analyses only of 
rocks which are unquestionably granite and basalt had been used, even 
if they had been fewer in number. Thus the analysis on page 66, recom- 
puted into the modal minerals (page 74), gives for granite a rock contain- 
ing but 17 per cent of orthoclase while it carries 35 per cent of oligoclase 
(Ab,,Anz5), besides biotite 6, muscovite 6, hornblende 2.5, quartz 31, 
magnetite 1.7, and ilmenite 0.3. The proportions of orthoclase to 
oligoclase are 17:35.5, making the rock a granodiorite, according 
to Lindgren’s original definition, but just over the line from quartz- 
monzonite. (The orthoclase here forms 32.4 per cent of the total feld- 
spar; Lindgren’s division line is 333 per cent.) The basalt, also, has for 
its modal feldspar oligoclase (Ab,oAn;.), and has 10.7 per cent orthoclase, 
besides augite 37, olivine 7.6, magnetite 5.8, ilmenite 0.73, and titanite 
2.8. Fora basalt the plagioclase is decidedly sodic, and the rock should 
rather be called a melanocratic orthoclase-bearing olivine-diorite. 
Comparing the modal minerals of these rocks with those in Clarke’s 
average igneous rock, the authors found that the latter was too low in 
quartz, assuming that the normative quartz in 84 granites (Q=32.8) 
is the same as the modal. But normative quartz is almost invariably 
higher than modal. If analyses of true granites and true basalts were 
used instead of granodiorite and orthoclase-bearing olivine-diorite, 
the authors’ and Clarke’s averages would be more nearly alike. It is 
true that the modes given for these rocks were calculated from the 
