File:Geological sketches, at home and abroad (1882) (14782229202).jpg

Identifier: geologicalsketch00geik (find matches)
Title: Geological sketches, at home and abroad
Year: 1882 (1880s)
Authors: Geikie, Archibald, Sir, 1835-1924
Subjects: Geology
Publisher: New York : Macmillan and Co.
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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tc. In a few cases the white marble has been employed byitself as a monolith in the shape of an obelisk, urn, or otherdevice ; but most commonly it occurs in slabs which havebeen tightly fixed in a framework of sandstone. Theseslabs, from less than one to fully two inches thick, aregenerally placed vertically ; in one or two examples theyhave been inserted in large horizontal sandstone slabs or through-stanes. The form into which the stone has beencut, and the position in which it has been erected, havehad considerable influence on its weathering. A specimen of the common white marble employed formonumental purposes was obtained from one of the marble-works of the city, and examined microscopically. It pre-sented the well-known granular character of true saccharoidmarble, consisting of rounded granules of clear transparentcalcite, averaging about t<j-q- of an inch in diameter (Fig.24, A). Each granule has its own system of twinlamellations, and interference colour-bands. The funda-
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164 GEOLOGICAL SKETCHES. (vin mental rhombohedral cleavage is everywhere well developed.Not a trace exists of any amorphous granular matrix orbase holding the crystalline grains together. These seemmoulded into each other, but have evidently no extra-ordinary cohesion. A small fragment placed in dilute acidwas entirely dissolved. There can be no doubt that thismarble must be very nearly pure carbonate of lime. The process of weathering in the case of this whitemarble presents three phases, sometimes to be observed onthe same slab—viz. superficial solution, internal disintegra-tion, and curvature with fracture. (1.) Superficial Solution is effected by the carbonic acid,and partly by the sulphuric acid of town rain. When themarble is first erected it possesses a well-polished surface,capable of affording a distinct reflection of objects placedin front of it. Exposure for not more than a year or twoto our prevalent westerly rains suffices to remove thispolish, and to give the surface a

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