Nature of the Earth's Interior. 163 



modified or corrected, so as to lead to the formation of a more and 

 more trustworthy opinion on the nature of those parts of our globe 

 which, from their position, must always remain inaccessible to our 

 powers of direct observation, it is imagined that a concise sketch of 

 the present actual state of our knowledge concerning the probable 

 constitution of the interior of the earth may prove interesting and 

 instructive. 



In treating this subject, we must first take into consideration 

 what has already been done in the way of direct examination of the 

 earth's substance in depth ; yet, when it is remembered that the 

 mean diameter of our planet is some 7912 miles, whilst the greatest 

 depth hitherto attained by man's direct exploration has not even yet 

 reached one mile from the surface downwards, the disproportion 

 appears so enormous as to render it self-evident in the pursuit of 

 this inquiry, especially as regards the more central portions of the 

 earth, that we must in the main rely upon the less direct evidence 

 furnished by calling in the assistance of the natural sciences. 



The direct examination of the exterior of the earth, even when re- 

 stricted to this depth, has, nevertheless, furnished us with many im- 

 portant data which can serve as a starting-point for this to a great 

 degree speculative inquiry, and to some of these attention will now 

 be directed. 



It must in the first place be remembered that all the rocks which 

 we encounter, and which compose so much of the solid exterior of 

 our globe as is actiially known to us, may be arranged under two 

 principal heads, viz. the volcanic or endogenous, i.e., those formed 

 within the body of the earth itself, and the sedimentary or exogenous, 

 i.e., those rocks formed, or rather reconstructed, upon its surface out 

 of the debris of previously existing rocks, arranged in beds or strata 

 by the mechanical action of water. 



It was until lately taken for granted by geologists, that the 

 lowest sedimentary strata, in their normal, or in a more or less 

 altered condition, rested directly upon granite, which was for a very 

 long time regarded as the rock foundation upon which they, in the 

 first instance, were deposited ; this rock being then looked upon as 

 the oldest of all, and even as representing the primeval or original 

 surface covering of the earth. Later researches have, however, 

 proved this hypothesis to be untenable, which is self-evident, siace 

 no instance of a granite has yet been met with in nature which, if 

 followed up, does not at some point or other break through or 

 disturb and alter, more or less, the stratified rocks in immediate 

 contact with it, so that it naturally follows that such stratified rocks 

 must have pre-e:^isted on the spot, or in other words, that they must 

 be older in geological chronology than the granite which came to 

 disturb them. 



In the present state of geological science, it is utterly impossible 

 for us to point out any variety of rock whatever as the one which 

 may have served as a foundation upon which the oldest sedi- 

 mentary beds were originally deposited ; in point of fact, the oldest 

 rocks which we know at present are sedimentary rocks, mostly in an 



