4-10 



was manifested by Dr. Hutton in deducing from thence the mean 

 density of the earth ; there remained one source of uncertainty in 

 their results, dependent on the specific gravity of the mountain, 

 which they assumed to be 2'5, but were aware that this might not 

 be so accurate as would be desirable in a standard to which the. den- 

 sity of the earth is directly compared. 



It was for this purpose that Mr. Playfair, with the assistance of 

 Lord Webb Seymour, undertook the present survey of Schehallien, 

 ascertaining, as accurately as they were able, the direction and in- 

 clination of the strata of which the mountain consists, and collecting 

 specimens of all the varieties of rock which they could discover, in 

 order to determine the specific gravity of each by direct experiment, 

 and thence to estimate the correction which it might be necessary to 

 introduce into Dr. Hutton's calculation. 



Schehallien, like all other mountains in its vicinity, was found to 

 consist entirely of primitive rocks in strata, which stretch in a direc- 

 tion from S.E. to N.W., and nearly vertical in position ; but in some 

 parts towards the base, they deviate as much as 15 from the per- 

 pendicular. The most elevated and by far the largest part of the 

 mountain was found to consist of granular quartz, extremely hard, 

 compact, and homogeneous, of which the specific gravity, by an 

 average of thirteen specimens, was nearly 2*64. Adjacent to this 

 mass, on each side, and next in quantity, is a micaceous schist, con- 

 taining hornblende ; and lower down toward the base appear strata 

 of granular limestone, highly crystallized, and containing mica. The 

 specific gravity of this part of the mountain, on an average of fifteen 

 specimens, was found to be 2'81. 



Beside these there are also veins or dykes of porphyry and green- 

 stone, which intersect the other strata at right angles, but bear a 

 very small proportion to the general mass of the mountain. 



With respect to the micaceous schist and limestone, which are 

 distinctly stratified in vertical planes, their continuity from one ex- 

 tremity of the mountain to the other may with confidence be pre- 

 sumed ; but this is not the case with respect to the granular quartz, 

 which may either be interposed between them, and continue with 

 them to an indefinite extent in length and depth, or may be of sub- 

 sequent formation, occupying only the summit of the mountain, and 

 not penetrating into the interior. 



Since the result of the calculation of the earth's density is mate- 

 rially affected by the difference of these suppositions, the author has 

 made an estimate according to each hypothesis. 



With the exception of the corrections thus obtained for the specific 

 gravity of the mountain, Mr. Playfair adopted the method employed 

 by Dr. Hutton, of dividing the whole mass of the mountain into a 

 certain number of vertical columns, formed by cylindrical sections, 

 so that their respective attractions on the plumb-line could with 

 great facility be computed upon the supposition of uniform density 

 throughout. 



Upon the former of the two hypotheses of Mr. Playfair, in which 



