350 Mr. C. Chree on some Applications of 



all over the earth — applies equally against such an hypothesis. 

 If, however, volcanic products be supposed to come from 

 several miles below the surface, I see no obvious reason why 

 they should not present similar characteristics everywhere. 

 No conclusive argument can well be based on the differences 

 observed in the sedimentary strata, because the conditions 

 under which such strata are deposited are obviously of a varied 

 character. 



In various passages of Professor Prestwich's discussion of 

 the state of the earth one is apt to be puzzled by his falling 

 into the practice, by no means uncommon in geological 

 writings, of employing physical terms with a view to oratory 

 rather than to exposition. For instance, he speaks of con- 

 traction a due to the yielding of the weaker lines in the crust, 

 when the tension caused by the excessive strain (and of which 

 the first order of movement is an index) overcomes the re- 

 sistance, and fractures and doubles up the strata ;" and he 

 adds, " Mountain-ranges are in fact the concluding term of 

 the stress which caused the deformation of the crust, and the 

 movements which at those times took place must have been 

 influenced by the greater energy of the strains then at work " 

 (p. 546) . It is difficult to see here what is intended to be cause 

 and what effect. In fact, while a number of terms are em- 

 ployed which in mathematics and physics have a fairly definite 

 meaning, I must confess my inability to form an adequate 

 conception of what is meant by the passage as a whole. 



Professor Prestwich refers (pp. 543, 544) to the hypothesis 

 of the late Professor E. Roche (in the reference to which a 

 misprint gives the year 1861 for 1881) as supplying some- 

 thing of the kind of earth he wants. Thus an examination 

 of Professor Roche's work* may be of some service. 



He supposes the earth to consist of a central nucleus or 

 "bloc," homogeneous but for a possible accumulation of 

 matter of greater density at the centre, and of a superficial 

 layer of lighter material. Of the nucleus, with the possible 

 exception of a small core of heavier matter, he says, " Sa den- 

 site calculee, de 7 a 7*5, indique qu'elle est metallique, sans 



doute formee de fer " The specific gravity of the heavier 



matter which may possibly exist at the centre is, he says, 

 " certainement bien inferieur [to 18], probablement 10 ou 12 

 (argent, plomb)/' p. 235. The outer layer or crust he sup- 

 poses to have a specific gravity about 3, and a thickness of 

 about one sixth the earth's radius. Between the cru*t and 

 the nucleus there exists, it may be everywhere or only locally, 



* Academie ...de Mvntpellier, Memoires de la Section des Sciences, tome 

 dixieme, 1880-84, pp. 221-264. 



