Notices respecting New Books. 213 



alterations of temperature and level, as the currents are disturbed, 

 producing changes in the motion of the liquid. Hence occur 

 changes of level on the Earth's crust over different areas of surface. 

 It is also remarked that "the extreme slowness with which 

 the accretion of fresh solid matter at the bottom of the crust 

 takes place [the molten substratum dissolving it nearly, but not 

 quite as fast, as it solidifies] would lead us to expect that its con- 

 stitution would, from the first, be crystalline and not vitreous. In 

 this respect it would differ from erupted igneous rocks, which are 

 in some cases at first vitreous, and in which a crystalline structure 

 is induced by subsequent metamorphism"' (page 78). 



IV. The crumpling and contortions of strata, and the inequali- 

 ties of the surface resulting from lateral stresses, are considered 

 in Chapters VII. to X.; and the existence of a yielding, fluid 

 substratum, holding water-gas in solution (that is, a magma in a 

 state of igneo-aqueous fusion), beneath a thin crust (about 25 miles 

 thick), is regarded as proved and in accordance with geological 

 evidence. The conditions of the existence of the water-substance 

 in a gaseous state, in both the primitive and the present stages cf 

 the Earth's history, are discussed in Chapter XI. It is explained 

 in Chapter XII. that the crust is not so flexible as to yield by 

 bending and folding only to lateral pressure ; but that the yielding 

 must take place through a crushing together and local thickening 

 of the crust. Chapter XIII. comprises the conditions of a dis- 

 turbed tract, with the formation of a mountain-range and its root, 

 downward protuberance, or equivalent subterranean mass, — also 

 the local results of denudation and sedimentation. Besearches 

 with the plumb-line, pendulum, and thermometer are severally 

 considered in their bearings on the condition of mountain-masses 

 and of the crust generally (Chapters XIV.-XVL). The attraction 

 of mountain-masses, and the rate of increase of temperature 

 w T ithin them, are both much less than would be the case if the 

 heavy, hot, molten liquid from below had risen into their anticlines. 



Oceanic areas and the suboceanic crust form the subject of 

 Chapter XVII. Oceans occupy depressions, definitely below the 

 spheroidal surface of the Earth ; and these have probably resulted 

 from a greater density of the crust (somewhat thicker there than 

 beneath the Continents) ; the soft substratum being not quite 

 so dense as that beneath the land. The permanency of the ocean- 

 basins is accepted. Prof. Darwin's speculation of the Moon haAing 

 broken away from the Earth some 50 million years ago is regarded 

 with favour. 



The thickness of the crust below the ocean — when the water is 



one mile deep may be £2*15 miles, 



two miles „ „ 49-34 „ 



three,, „ „ 124*90 



Its density is also calculated (2-80 to 2*95). At the rare depth of 

 five miles the density might be so great as to allow the crust to 

 lose heat more rapidly and to sink slowly, — as, for instance, near 

 Japan, where the frequent earthquakes may be due to this cause. 



Phil Mag. S. 5. Vol. 29. No. 177. Feb. 1890. R 



