Page:Proceedings of the Royal Society of London Vol 1.djvu/430

Rh 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 density 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 inclination 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 direction from SE. 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, the'u' continuity from one extremity of the mountain to the other may with conﬁdence be presumed ; 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 subsequent 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 materially 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 then 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