ON TERTIARY GABBROS ETC. IX SCOTLAND AND IRELAND. 49 



6. On the Gabbros, Dolerites, and Basalts, of Tertiary Age, in 

 Scotland and Ireland. By Professor John W. Jfdd, F.E.S., 

 Sec. G.S. (Read December 16, 1885.) 



[Plates IV.-VII.] 

 I. Introduction. 

 II. Geographical Distribution. 



III. Geological Age. 



IV. ^Nomenclature of the Rocks. 



V. Minerals of which the Rocks are composed. 

 VI. Proportions of the several Minerals in the Rocks. 

 VII. Structures of the Rocks. 

 VIII. Geological Relations of the Rocks exhibiting the several Structures. 

 IX. Origin of the Structui'es. 



X. Principles which have governed the Crystallization of the Rocks. 

 XI. Alterations which the Rocks hare undergone. 



§ 1. Action of surrounding Magma upon Crystals. 

 § 2. Action of Solvents under Pressure (Schillerization). 

 § 3. Action of Steam and other Gases at the Surface. 

 § 4. Action of Atmospheric Agents (Weathering). 

 § 5. Effects of Crushing Movements in Rock-masses. 

 XII. Summary of Results. 



I. Introdijction. 



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 ex- 

 planation of the undoubted fact that the igneous rocks of Tertiary 

 and recent date, as a general rule, exhibit far less perfectly crystal- 

 line characters than those which belong to the older geological 

 periods. Cceteris 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 alter- 

 ation, 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 the unaltered 

 type may easily escape recognition. 



Now as the result of the more common occurrence of the crystalline 

 Q.J.G.S. No. 165. B 



