part 1] AJfNIYERSARY ADDRESS OF THE PRESIDENT. XCV 



Raasay and the Shiant Isles in the north, or again with the 

 analcime-dolerite (crinanite) sills of Southern Arran and the ortho- 

 phyre with riebeckite and segirine on Holy Isle. If we tr}^ to push 

 the examination farther afield, where the only Tertiary rocks pre- 

 served are in the form of dykes, we find very few trustworthy 

 data. A number of dykes in Argyllshire and elsewhere, belonging 

 to very characteristic alkaline types, have sometimes been claimed 

 as Tertiary, but I have assigned them, upon a balance of evidence, 

 to a Permian age. When, however, we pass beyond the bounds of 

 Scotland, southwards and westwards, we meet with a few scattered 

 occurrences which have a special interest. The nepheline-basanite 

 of Butterton, the phonolite of the Wolf Rock, and the peculiar 

 segirine-bearing type of Rockall are reasonably regarded as 

 Tertiary. They are of much more strongly alkaline composition 

 than the known rocks of this age in Northern Britain, and find 

 their nearest relatives in the Cape Verd Isles and other island 

 groups far to the south. If then we are to distinguish among 

 the 'younger' igneous; rocks of this part of the world a strongly 

 alkaline North Atlantic region, connected with the collapse of con- 

 siderable tracts in mid-ocean, and a less strongly alkaline Arctic 

 region, including the Brito-Icelandic Province of Judd, we can 

 point to one more important dividing line crossing the British 

 area. It is in the nature of the case, however, that much of the 

 evidence by which we might follow out this suggestion is buried 

 beneath the waves. 



Here I must bring my rough summary of igneous action in 

 Britain to a close. Apart from personal shortcomings, it has 

 suffered from necessarj^ condensation and sometimes from the 

 insular method of treatment imposed by considerations of time and 

 space. I have tried to show that at different periods of its history 

 our area has been differently mapped out into petrographical 

 provinces, and this in a manner quite definitely related to the larger 

 displacements of the earth's crust. The inference seems just that 

 the distribution of crustal stress is a dominant factor 

 in determining the petrographical facies of igneous 

 rocks. One way in which this factor may operate has been 

 suggested, and I venture to think that in an}- theoretical discussion 

 of the question a place must be found for that intercrustal creep 

 of alkaline magmas which 1 have specially emphasized. That it is 



