RESULTS OF THE EARTH'S CONTRACTION. 737 



(On this and related subjects, see Daubree, Exp. Synth, relatives aux Meteorites, 

 Comptes Rendus, lxii., 1866, and Smithson. Ann. Rep. for the year 1868.) 



That the dolerytic or basic rocks should have been the most abundant, in the earth's 

 liquid interior, is indicated, as Hunt has observed, by the fact that nearly all the lime 

 of limestones must then have been in the condition of silicates, making, probably, the 

 feldspar, labradorite, and forming, with iron and the magnesia now in magnesian lime- 

 stones, augite, hornblende, or chrysolite. 



The rock of the true crust must be coarsely crystalline, and, in this respect, unlike 

 ordinary doleryte ; for a coarsely crystalline structure is a necessary consequence of 

 extremely slow cooling. Which of the two minerals, augite and hornblende (essentially 

 alike in constitution, but unlike in crystallization), would have been formed, there are 

 not yet facts to decide. 



4. Constitution of the JEarth's Nucleus. — The large proportion of 

 oxyd of iron, in igneous rocks and the Archaean terranes, suggests that 

 the increase of density in the earth toward the centre, shown to exist 

 by its specific gravity, 5-5 to 6, may be due, so far as it is not owing 

 to increased density below from pressure, to the presence of iron, 

 either pure or in combination. All our platinum and gold come from 

 the supercrust and its infiltration-veins, and hence were derived from 

 the outer part of the true crust. They probably reached this exterior 

 position, through combinations under the extreme heat ; and most that 

 existed in the sphere may have thus escaped. If iron be the chief 

 material of the nucleus, and the specific gravity be mainly due to it, 

 supposing no increase of density below, the mass of the globe should 

 be two-thirds iron, and this would bring the iron to within 500 miles 

 of the outer surface, so that the nucleus, in such a case, might be 

 nearly all iron. The iron meteorites which have reached the earth, 

 appear to favor the above view. 



5. Cleavage Structure in the Earth's Crust. — The prevalent north- 

 east and northwest courses of trends, the curves in the lines varying 

 the direction from these courses, and the dependence of the outlines 

 and feature-lines of the continents and oceanic lands upon these 

 courses (p. 29), are the profoundest evidence of unity of development 

 in the earth. Such lines of uplift are lines of fracture, or lines of 

 weakest cohesion ; and therefore, like the courses of cleavage in 

 crystals, they show by their prevalence some traces of a cleavage- 

 structure in the earth, — in other words, a tendency to break in two 

 transverse directions rather than others. 



Such a cleavage-structure would follow from the mode of origin of 

 the earth's crust. The crust has thickened by cooling, until now 

 scores of miles through ; and very much as ice thickens, — by additions 

 to its lower surface. Ice takes on a columnar structure, perpendicular 

 to the surface, in the process, so as often to break into columns, on 

 slow melting. The earth's crust contains as its principal ingredient 

 one or more kinds of feldspar, all cleavable minerals ; and, as crystals, 

 47 



