306 SCIENTIFIC EECOKD FOR 1882. 



the density at the centeroftbe earth is 9.59 under a pressure of 2,500,000 

 atmospheres, and he thinks it probable that the magma beneath the 

 outer crust consists of felspathic materials, passing k)wer down into au- 

 gitic, and finally, at the center, into a magnetic magma. 



"The next ijroblem to be discussed relates to the manner in which 

 the heat and the gravitation of the earth have produced the elevations 

 and depressions and pitckerings of the surface. To explain this it is 

 generally thought that, as the cooling of the earth proceeded, the 

 interior retreated from the solidified crust, and that the latter became 

 crumpled and contorted by the lateral pressure. The author has cal- 

 culated that the pressure available for this purpose would be equal to 

 that of a column of rock of the surface density, having the same section 

 as the stratum, and 2,000 miles in length — a pressure equal to 830,200 

 tons on the square foot, and more than sufficient to perform the opera- 

 tions assigned to it. • 



"Volcanic eruptions probably arise from liquid masses of the substra- 

 tum gaining access to the surface, and we must conceive that the water 

 which accompanies all volcanic phenomena must be present in the mag- 

 ma of the substratum. 'We may look upon the state of igneoaqueous 

 solution, ' observes the author, ' as one in which the water-substance is 

 in a gaseous state, and the combination between the water-substance 

 and the rock is probably of that kind which has been termed " occlusion" 

 of gas by a liquid. An examination of the amount of contraction which 

 would have produced the existing inequalities of the earth's surface 

 shows that the ocean basins are not the result solely of depressions in 

 the upper surface only of a crust of uniform density, but that they are 

 due to the greater density and general depression of the suboceanic 

 crust.' 



''According to the author, volcanic energy is the cause of the compres- 

 sion of the earth's crust. Thus he reverses the theory of Mallet, which 

 makes volcanic energy the result rather than the cause of compression, 

 and he shows that the utmost conceivable amount of heat capable of 

 being obtained by his theory is inadequate to the purpose assigned to 

 it. He considers, moreover, that the geographical distribution of vol- 

 canoes is better explained on the supposition of a thin crust and fluid 

 substratum than upon any other. 



" 'Their linear arrangement points to their being situated along great 

 systems of fissures ; and such systems of fissures are indicative of a thin 

 crust. Fissures which run for long distances in nearly straight courses 

 point either to a movement perpendicular to the fissured surface or else 

 to a rending pressure within the fissure itself; while on the other hand 

 fissures which are caused by contraction in a direction parallel to the 

 earth's surface would divide up an area into polygonal fissures. The 

 former arrangement of the fissures accords best with the distribution - 

 of volcanic ranges and suggests a thin crust.' 



" Volcanic regions are either oceanic or appertaining to the coast, and 



