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NATURE 



[September 7, igu 



merely prove that it is not a final expression of the whole 

 truth, and may point the way to its revision and correc- 

 tion. 



Take, tor instance, our provisional law of the distribu- 

 tion of the two branches of igneous rocks in defined regions. 

 It has been objected that leucitic lavas, having therefore 

 very decided alkaline or Atlantic affinities, are known at 

 several places within the limits of the main Pacific region, 

 where they are associated with andesitic and other calcic 

 rocks. Now, the only area for which we have anything 

 like full information is the island of Java. Here, according 

 to Verbeek and Fennema, the great plateau-lavas of 

 Tertiary age are exclusively of andesitic types, and the 

 same is true of the long chain of 116 volcanic centres, 

 which represent the later revival of activity. As against 

 this record there are five volcanoes, long extinct, which at 

 one stage erupted leucitic lavas. Whether we suppose these 

 to be aberrant derivatives from an andesitic magma, or, 

 much more probably, an incursion from the neighbouring 

 alkaline region, it seems reasonable to regard these very 

 exceptional occurrences as of the second order of import- 

 ance, and to set them aside in a first attempt to reduce the 

 facts to order. 



The discovery of various alkaline rocks on Hawaii, 

 Samoa, Raratonga, Tahiti, and other islands in the midst 

 of the Pacific Ocean raises, 1 think, a different question. 

 So far as is known, these rocks are not found in close 

 association with characteristic calcic types. Suess's 

 masterly discussion of all the geographical and hydro- 

 graphical data hitherto obtained makes it clear that an 

 Atlantic as well as a Pacific element of structure enters 

 into some parts of the Pacific basin. In certain areas, 

 such as the Galapagos Archipelago, the coming in of the 

 Atlantic regime is quite clearly reflected in an alkaline 

 facies of the igneous rocks, and such exceptions are there- 

 fore of the kind which go to prove the rule. Both Max 

 Weber and Lacroix have expressed the opinion that the 

 andesitic branch of rocks is characteristic of the border of 

 the great Pacific basin rather than the interior. It is 

 possible that further knowledge may justify this conclusion, 

 and still only confirm the relation which is claimed between 

 the two tectonic types and the two petrographical facies. 

 Meanwhile, we find clear evidence elsewhere that vertical 

 subsidence and lateral thrust have sometimes occurred in 

 the same region or in the same petrographical province : 

 nor need we go far from home to learn that the complexity 

 of structure thus implied is nrcompanied by a correspond- 

 ing peculiarity of petrographical facies. 



The North British Tertiary Province. 



In order to illustrate this point in a concrete instance, I 

 will discuss very briefly a single petrographical province, 

 viz. that which occupied the northern part of Britain in 

 early Tertiary times. Prof. Judd has regarded this as 

 forming part of a larger " Brito-Icelandic province " ; but, 

 while recognising many affinities between our rocks and 

 those of higher latitudes, I think that the North British 

 area possesses enough individuality to be more properly 

 treated as a distinct unit. The record of igneous action 

 here is exceptionally complete and well displayed. Our 

 knowledge of it is derived, in the first place, from Prof. 

 Zirkel, Sir Archibald Geikie, and Prof. Judd, and more 

 recently from the detailed work carried out by the Geo- 

 logical Survey of Scotland. This latter is, as regards the 

 Isle of Mull, still in progress, 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 Yorkshire ; but the chief theatre of 

 igneous activity was the sunken and faulted tract of the 

 Inner Hebrides, between the mainland of Scotland, on the 

 one hand, and the Arch.-ean 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, am the plutonic centres of Skye, Rum, Ardna- 

 murchan, and Mull. Farther 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 Ailsn lie on a parallel line only a 

 little farthei 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 

 Prof. 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 rigime. 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 

 in the neighbourhood of those special centres 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 

 Rum) contemporaneous gneissic structure in the plutonic 

 masses themselves. The disturbances 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 reference 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 com- 

 plex 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 

 indicated by the diversion of the very numerous dykes from 

 their normal north-westerly direction to a radial arrange- 

 ment 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 recognisi-d 

 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 move- 

 ments 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 petro- 

 graphical 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 rang- 

 ing from ultrabasic to thoroughly acid. The ultrabasic 

 rocks, as developed in Rum and Skye, have a lime-felspnr 

 as one of their chief components : there are no picrites (in 

 the original sense of Tschermak) or other alkaline types. 

 The eucrite group, found in Rum, Ardnamurchan, and the 

 Carlingford 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 iess acid is usually augitic, while the more 

 acid, found in Arran. St. Kilda, and the Mourne Moun- 

 tains, 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, 

 of mean acidity being absent. This has nn int. 

 [uence. In many places a granite magma, in- 



2184, vol. 87] 



