376 TRANSACTIONS OP SECTION C. 



exceptions in experience can be applied to the construction of a system that may 

 be called natural.' I hold, on the contrary, that such a science as Geology can 

 be advanced only by the inductive method, which implies provisional hypotheses 

 and successive approximations to the truth. A generalisation which brings 

 together a mass of scattered observations, and endows them with meaning, is 

 not invalidated by the discovery of exceptions. These merely prove that it is 

 not a final expression of the whole truth, and may point the way to its revision 

 and correction. 



Take, for instance, our provisional law of the distribution 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 asso- 

 ciated 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 importance, 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, I think, a different 

 question. So far as is known, these rocks are not found in close association with 

 characteristic calcic types. Suess' masterly discussion of all the geographical and 

 hydrographical 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 therefore 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 accompanied by a corresponding 

 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. Professor 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 bei more properly treated as a 

 distinct unit. The record of igneous action here is exceptionally complete and 

 we 1 } displayed. Our knowledge of it is derived in the first place from Professor 

 Zirkel, Sir Archibald Geikie, and Professor Judd, and more recently from the 

 detailed work carried out by the Geological 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 

 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 centres of Skye, Rum, Ardnamurchan, and Mull. Further 



