Joty—An Estimate of the Geological Age of the Earth. 43 
These may be tabulated as follows (the references are to the pages of the work 
referred to) :— 
K.0. Na,20O. 
Eruptive rocks— 
Mean of 19 Liparites, Trachytes, Basalts, &c. (p. 35), 3:03 4:37 
Plutonic rocks— 
Mean of 18 Granites (p. 78), : : - : 4:26 3°80 
9 46 Syenites (pp. 106-126), : : . 5°60 6°80 
30 25 Diorites (pp. 140, 141), 6 5 : 1°86 3°82 
99 6 HEssexites (p. 172), 3 : 2°49 4:68 
5 8 Theralites and Shonkinites (p. 176), . 3°86 4:98 
99 14 Gabbros (p. 151), 9 : : : 0-77 2°62 
Mean of eruptive and plutonic rocks, Bali 4°44 
Also the following dynamically altered eruptive rocks :— 
Mean of 5 PorphyroidsandSerecite-gneisses (p.440), 6:44 1-68 
- 12 Mica-gneisses (p. 468), : : 5 4:24 2:99 
59 4 Amphibole-gneisses (p. 484), : ; 3°14 5:13 
5 6 Pyroxene and Augite-gneisses (p. 486), 2°63 2°66 
99 13 Halleflinte-gneisses (p. 493), : . 2°18 2°68 
09 10 Amphibolites (p. 515), . : : . 2°02 3°96 
99 6 Eklogites (p. 520), : . : : 0°35 1°85 
Mean of metamorphic eruptive rocks, 3°00 2°99 
Mean ofall, . : : ; : 3°06 3°72 
With reference to the last division of rocks—the metamorphic eruptives— 
Rosenbusch admits the old standing difficulty of distinguishing between the 
altered sediments and the altered eruptives. ‘Thus Gneisses derivable from sedi- 
ments give just such chemical proportions as appear in those referred to eruptive 
origin. There exists no sure criterion for classing a Gneiss according to its origin 
(p. 472, loc. cit.) 
This is a well-known difficulty, and has been the subject of much research and 
speculation. We dare not do more than suggest here that in doubtful cases the 
general law of the alkali ratios of eruptives and sedimentaries, where this admits 
of application, should carry weight with petrologists. 
The uncertainty referred to may, of course, affect the estimated composition 
of the crust-rock and of the siliceous sedimentaries. Examination will, however, 
show that the uncertainty being confined to a couple of groups only, can probably 
affect the final averages but little im either case. 
