362 



SCIENCE 



[N. S. Vol. XXXIV. No. 873 



as regards the Isle of Mull, still in prog- 

 ress, and will doubtless when completed 

 throw additional light on some questions 

 still obscure. 



The province includes all western and 

 southern Scotland, with the northern part 

 of Ireland, and extends southward as far 

 as Anglesey and Yorltshire, but the chief 

 theater of igneous activity was the sunken 

 and faulted tract of the Inner Hebrides, 

 between the mainland of Scotland, on the 

 one hand, and the Archaean massif of the 

 Outer Isles, on the other. It is here that 

 the volcanic accumulations attain their 

 greatest thickness, and here, closely set 

 along a N.-S. line, are the plutonic centers 

 of Skye, Rum, Ardnamurchan and Mull. 

 Farther south are the volcanic plateau of 

 Antrim and the neighboring plutonic cen- 

 ters of the Mourne Mountains and Carling- 

 ford, while the two centers of Arran and 

 that of Ailsa lie on a parallel line only a 

 little farther east. In addition it is clear 

 that igneous activity extended westward 

 over a tract now submerged under the At- 

 lantic, and here too plutonic centers 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 Ire- 

 land. 



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 ex- 

 ample 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 particu- 

 larly in the neighborhood of those special 

 centers which were marked at one stage by 



plutonic intrusions. The evidence is seen 

 in sharp anticlinal folding; sometimes also 

 in crush-brecciation along quasi-horizontal 

 bands and (in Eum) contemporaneous 

 gneissic structure in the plutonic masses 

 themselves. The disturbances in Mull, as 

 described by Mr. Bailey, are especially in- 

 teresting. The whole eastern coast-line of 

 the island is determined by a system of 

 concentric curved axes of folding, aflfecting 

 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 center of the island, 

 and a somewhat similar arrangement is 

 found on the east side of the Skye center. 

 All these facts go to show that in the dis- 

 trict surrounding any one of the special 

 centers there was developed a complex 

 system of stresses, which found relief 

 partly in igneous action, partly in dis- 

 placements 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 indicated by the di- 

 version of the very numerous dykes from 

 their normal northwesterly direction to a 

 radial arrangement about the special cen- 

 ters, 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 centers, usually in- 

 tersecting the plutonic rocks and a small 

 fringing belt, and constantly dipping in- 

 wards. Such sheets occur in immense num- 

 bers in the gabbro mountains of Skye and 

 Mull, and they are to be recognized also in 

 Rum and Ardnamurchan. 



It is plain then that this province ex- 

 emplifies at once the two tectonic types dis- 

 tinguished by Suess. There has been a 

 general subsidence, affecting the area as a 



