HISTORY OF CRYSTALLINE ROCKS. 19 
the pyroxenic element, occurs in predominant quantity. An example of this is seen in a 
coarsely granitoid chrysolitic aggregate, exposed with the same characters, over an area 
of many hundred square feet, on Montarville. The chrysolite in this rock is in irregular 
crystalline masses from five to ten millimetres in diameter, and was separately analyzed, 
as was the black pyroxene, found in still larger and well defined crystals in the mass, and 
also the feldspathic element, selected as carefully as possible. For an analysis of the rock 
as a whole, it was attacked in fine powder successively by dilute sulphuric acid and by 
a weak solution of soda, the portions thus dissolved being analyzed separately, as well as 
the insoluble residue. The relative proportions of these being 55.0 per cent of the former 
and 45.0 of the latter, it became possible to calculate the composition of the rock as a 
whole. 
§ 30. In the following table, I is the composition of the feldspar ; II, the pyroxene ; 
III, the chrysolite; IV, the soluble portion (55.0 per cent), chiefly chrysolite; V, the 
insoluble portion (45.0 per cent); VI, the rock as a whole, including an undetermined 
amount of titanic oxyd with the iron-oxyd. For the purposes of comparison we give 
under VII the composition of the supposed basic magma of the earth’s interior, as deduced 
by Bunsen from the mean of several analyses of basic eruptive rocks, and under VIII the 
composition of the same, as calculated by Durocher, who, however, admits a range in pro- 
portions through geologic time which includes the figures adopted by Bunsen. The last 
five analyses are necessarily calculated for one hundred parts, and the whole of the iron is 
represented as ferrous oxyd, although an unknown proportion exists in a higher state of 
oxydation. 







Ty IDE IIT. IV 
Silay cejsieewae rete ts 53.10 49.40 37.17 37.3 
Alumina .......: 26.80 6.70 == 3.0 
Times enr ce 11.48 21.88 _— — 
Magnesia......... 0.72 13.06 39.68 33-5 
Ferrous oxyd ..... 1.35 7.03 22.54 26.2 
SOA ere ere me (4 24: 0.74 _ — 
Batashecceeerctre 0.71 = = = 
SViolsitalle years eu-tets-t+7 0.60 0.50 _— = 
99.00 100.11 99.39 100.0 
V. VI. VIL. VIII. 
Siren eos 49.35 42.70 48.47 51.5 
Aluminarcscee...e 18.92 10.16 14.7 16.0 
MER aaadonopocous alse 8.27 11.87 8.0 
Magnesia ........, 6.36 21.29 6.89 6.0 
Ferrous oxyd...... 4.51 16.45 15..38 13.0 
ATK ATIOS vce «nieces 2.50 1.13 2.61 4.0 
100.00 100.00 100.00 ae 
§ 31. The process which has thus given rise in parts of a mountain mass of dolerite 
to considerable areas of a rock containing over 21.0 of magnesia, and more than one half 
its weight of chrysolite, and in other parts of the same mass to an aggregate of pyroxene 
and labradorite, almost, and in some cases wholly, destitute of chrÿsolite, is readily ex- 
plained if we admit a separation from a still fluid mass of the previously crystallized and 
