Formation of Continents and Oceante depressions. 165 
are just the rocks that are likely to have been formed over the 
earth’s surface after the action on the crust of the foul atmos- 
pheric vapors that settled upon it as it began to solidify. If 
there were mainly doleritic material and other labradorite mix- 
tures in that crust, the result of the conflict would have been a 
removal of part of the bases and the liberation of silica, making 
free quartz and quartz-bearing rocks. 
ain, the general fact that the doleritic rocks, and even 
most trachytic, contain disseminated grains of uncombined 
oxide of iron in the form of magnetite (Fe,O,) adds to the 
strength of the argument against the general diffusion of quartz, 
seminated magnetite and hornblende; and from this extreme 
there is a shading off in trachytic rocks toward dolerites, 
syenites, or hyposyenites. It should be here understood that 
augite and hornblende are essentially identical in chemical con- 
ifferi izati ornblende has 
bases and silica frequently 1:2} as in the feldspar oligoclase, 
instead of 1:2, as in augite, and this ma be one reason for its 
occurrence by preference in the trachytes, in which oligoclase 
and orthoclase predominate, and that of augite in the olerites, 
in which labradorite is the predominant feldspar. In labrador- 
ite this ratio is 1:14, and in andesine 1:2. Mix 
labradorite and oligoclase, which constitute the base of some 
doleritic rocks (melaphyres, in which the silica constitutes over 
55 per cent), would have 1:2 for this oxygen ratio when the 
proportion of labradorite to oligoclase was . 
Th the first solidified crust 1s 
ie") 
= 
oO 
fl 
ved 
2 
B 
or 
® 
= 
Lan | 
ee 
=) 
eis} 
eh 
ie") 
aS] 
oe 
So 
é 
° 
=r 
hypersthenite and diabase, which approach ks ete And 
besi es, there are diorite, consisting of hornblende 
or oligoclase, and hyposyenite, consisting of hornblende and 
orthoclase, 
