DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 151 



however, of being a solitary instance, it is only one of hundreds of similar intrusions, 

 which can be connected with the general body of granitoid and porphyritic rocks, and 

 which put the relative ages of the two groups of rock beyond any further doubt. 



Boue, who knew the geology of some of the extinct volcanic regions of Europe, recog- 

 nised the similarity of the Scottish masses to those of the Continent, and classed the 

 acid rocks as " trachytes." He saw in each of the volcanic areas of the west of Scotland 

 a trachytic centre, and supposed that the more granitoid parts might represent the centres 

 in the European trachytic masses. He traced in imagination the flow of the lava-streams 

 from these foci of volcanic activity, distinguishing them as products of different epochs 

 of eruption, among the last of which he thought that the trachytic porphyries might have 

 been discharged. He admitted, however, that his restoration could not be based on the 

 few available data without recourse to theoretical notions drawn from the analogy of other 

 regions.* 



In the careful exploration of the central region of Skye made by Von Oeynhausen and 

 Von Dechen, these able observers traced the boundary between the " syenite " and the 

 " hypersthene rock "; and as they found the former lying underneath the latter, they seem 

 naturally to have considered it to be the older protrusion of the two.t Principal Forbes 

 came to a similar conclusion from the fact that he found the dark gabbro always over- 

 lying the light-coloured felspathic masses. | Professor Zirkel also observed the same 

 relative position, and adopted the same inference as to the relative age of the rocks. § 

 Professor Judd followed these writers in placing the acid rocks before the basic. He 

 has supposed the granitoid masses to form the cores of volcanic piles probably of 

 Eocene age, through and over which the extrusions of gabbro and the eruptions of the 

 plateau-basalts took place. || 



Among the protrusions of acid rocks in the Tertiary volcanic areas of Britain four 

 distinct types of structure may be noted, viz. (l) bosses, (2) sills or intrusive sheets, 

 (3) veins and dykes, and (4) superficial lava-streams. Of these the first three belong 

 entirely to the underground operation of volcanism, the last is the only one which reveals 

 the outflow of material at the surface. 



1. Bosses. 



These are irregular protrusions varying in size from knobs measuring only a few 

 square yards up to huge masses many square miles in extent, and comprising groups of 

 lofty hills. As a rule, their outlines are markedly irregular. Beneath the surface they 

 plunge down almost vertically through the rocks which they traverse, but in not a few 

 instances their boundaries are inclined to the horizon so that the contiguous rocks seem 



* Essai Gdologique sur VEcosse, pp. 291, 322, 327. 

 t Karsten's Archiv, i. p. 82. 

 X Edin. New Phil. Jour., xL (1846) p. 84. 



§ Zeitsch. Deutsch. Geol. Gesellsck, xxiii. (1871) pp. 90, 95. He says that the gabbro seems to be the younger rock, 

 so far as their relations to each other can be seen. 

 || Quart. Jour. Geol. Soc, xxx. p. 255. 



