PRESIDENTIAL ADDRESS. 377 



south are the volcanic plateau of Antrim and the neighbouring plutonic centres 

 of the Mourne Mountains and Carlingford, while the two centres of Arran and 

 that of Ailsa lie on a parallel line only a little further east. In addition it is 

 clear that igneous activity extended westward over a tract now submerged under 

 the Atlantic, and here too plutonic centres were not wanting. One is exposed in 

 St. Kilda, 50 miles west of the Outer Hebrides, and another has been inferred 

 by Professor Cole from a study of the stones dredged on the Porcupine Bank, 

 150 miles west of Ireland. 



The connection of igneous action in this province with the subsidence of 

 faulted blocks of country is too plain to be missed ; and so far, excepting the 

 tendency to a definite alignment of the foci of activity, we seem to be dealing 

 with a typical example of the Atlantic regime. The actual tectonic relations 

 are, however, of a more complex kind, and undoubtedly involve the element of 

 lateral thrust as well as vertical subsidence. This is more particularly so in the 

 neighbourhood of those special centres which were marked at one stage by 

 plutonic intrusions. The evidence is. seen in sharp anticlinal folding; some- 

 times also in crush-brecciation along quasi-horizontal bands and (in Rum) contem- 

 poraneous gneissic structure in the plutonic masses themselves. The distur- 

 bances in Mull, as described by Mr. Bailey, are especially interesting. The 

 whole eastern coast-line of the island is determined by a system of concentric 

 curved axes of folding, affecting all the rocks up to the Tertiary basalts, which 

 are in places tilted almost vertically. The curved axes are disposed with refer- 

 ence to the plutonic centre of the island, and a somewhat similar arrangement is 

 found on the east side of the Skye centre. All these facts go to show that in 

 the district surrounding any one of the special centres there was developed a 

 complex system of stresses, which found relief partly in igneous action, partly in 

 displacements of the solid rocks. Nor were the effects confined to the plutonic 

 phase. At a later epoch the influence of these local stresses is sometimes indi- 

 cated by the diversion of the very numerous dykes from their normal north- 

 westerly direction to a radial arrangement about the special centres, as is seen 

 partly in Skye and more strikingly in Rum. There are also local groups of 

 dykes developed only in these districts, and these again sometimes have a radial 

 arrangement. More remarkable are the groups of inclined sheets which are 

 found about the same centres, usually intersecting the plutonic rocks and a small 

 fringing belt, and constantly dipping inwards. Such sheets occur in immense 

 numbers in the gabbro mountains of Skye and Mull, and they are to be recog- 

 nised also in Rum and Ardnamurchan. 



It is plain then that this province exemplifies at once the two tectonic types 

 distinguished by Suess. There has been a general subsidence, affecting the area 

 as a whole but not all parts equally, and with this we must connect those groups 

 of igneous rocks which have a wide distribution throughout the province. But 

 there have also been movements in the lateral sense more strictly localised and 

 more sharply accentuated, and to these belong evidently the plutonic rocks with 

 various other groups which are their satellites. I have pointed out these facts 

 elsewhere, but failed to follow out the logical conclusion on the petrographical 

 side. Influenced by the strongly marked characters of the plutonic series, I 

 assigned the North British Tertiary rocks, not without some misgivings, to the 

 calcic or Pacific region. Suess, having regard probably to the broader tectonic 

 features rather than to petrographical data, has included our area in the Atlantic 

 region. 



Concerning the calcic facies of the plutonic rocks there can be no question. 

 They constitute a well-defined ' rock-series,' intruded in order of decreasing 

 basicity, and ranging from ultrabasic to thoroughly acid. The ultrabasic rocks, 

 as developed in Rum and Skye, have a lime-felspar as one of their chief com- 

 ponents : there are no picrites (in the original sense of Tschermak) or other alka- 

 line types. The eucrite group, found in Rum, Ardnamurchan, and the Carling- 

 ford district, is also characterised by a felspar near anorthite. Gabbros are 

 represented at nearly all the several centres, and in Arran they are accompanied 

 by norites. The granites and granophyres fall into two sub-groups. The less 

 acid is usually augitic, while the more acid, found in Arran, St. Kilda, and the 

 Mourne Mountains, carries hornblende and sometimes biotite. 



This series is known in various provinces of Pacific facies. A peculiarity of 

 it is that it is a broken series, types of mean acidity being absent. This has an 



