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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 Excerpt: ... basic sills of this great group, presenting a very general community of petrographical characters, show also considerable uniformity as regards habit and appearance in the field. In thickness there is great variation, the single sills ranging from a few inches to 200 feet or more. Apart from this, the chief difference observable among the sills in the field depends upon the degree of development of columnar jointing in the rocks. This character may be wholly wanting, or may occur with varying development and regularity. Where it is absent, the sills, and especially those of moderately large dimensions, very often show a certain platy or rudely laminated structure parallel to the upper and lower surfaces and most marked in the neighbourhood of those surfaces. This is often found also in association with the columnar jointing. It imparts a certain quality of fissility to the sill, but is not related to anything in the intimate structure of the rock, and must be regarded as an effect of contraction. Conspicuous columnar jointing in the sills seems to have a welldefined geographical distribution in the island. It is very prevalent and often very perfectly developed in the north, but gives place, as we pass southward towards the central tract, to a rude division into blocks, frequently showing the quasi-horizontal platy structure already mentioned. When most clearly marked, the columns are as a rule at right angles, or nearly so, to the surfaces of the sills, and therefore make only a small angle with the vertical, depending on the dip of the sills themselves. Irregularities are, however, found in some places (Plate IX.). The columns extend from top to bottom of even the thickest sills, though they may be broken at intervals by cross-joints. Some of the finest ...

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