On the Gabbros, Dolerites, and Basalts, of Tertiary Age, in Scotland and Ireland

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Abstract
Among igneous rocks of every variety of chemical composition, the most highly crystalline types appear to have been formed in those cases only where the liquid magmas have cooled down with extreme slowness and under enormous pressure. Such conditions of slow cooling and great pressure it would seem must have existed when the rock has consolidated at great depths within the earth's crust; and hence it follows that the highly crystalline rocks now seen at the surface have been exposed through the removal, by denudation, of the vast deposits which originally overlay them. Bearing these principles in mind, it is not difficult to find an explanation of the undoubted fact that the igneous rocks of Tertiary and recent date, as a general rule , exhibit far less perfectly crystalline characters than those which belong to the older geological periods. Cæeteris paribus , the older a deep-seated igneous rock is, the greater will be the chance of its being exposed at the surface, through the removal by denudation of the mass of materials under which it was originally formed. Further than this, it must be remembered, as Allport and others have so well shown, that the older a rock-mass, the greater is the probability that its constituent minerals will have undergone alteration, from the action of those chemical forces which are everywhere at work within the earth's crust. In this way the aspect, the structure, and even the mineralogical constitution of a rock may be so completely changed that its real relations with

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