.8^,. THE SEQUENCE OF PLUTONIC ROCKS. 291 



With these general and more or less disconnected remarks, let us 

 now consider some cases in which a definite sequence of igneous 

 rocks has been established. That we are very far from being able to 

 explain all the facts must, of course, be admitted at once, but at the 

 same time we may be allowed to remark that there are not wanting 

 indications that some of these facts are due to the operation of those 

 causes to which attention has been drawn. 



We commenced this article by referring to volcanic rocks. 

 Now the magmas from which such rocks are derived must either 

 exist independently in the interior of the earth, or else be formed by 

 differentiation from some pre-existing magma, or in some other and 

 unknown way. In any case, seeing that plutonic rocks originate in 

 the earth's interior, and represent, so to speak, the subterranean 

 manifestations of volcanic energy, it seems probable that a careful 

 study of the relations of complex plutonic areas will throw light on 

 the origin of different magmas. It is possible that some plutonic 

 areas may represent consolidated magma-basins that have never 

 been tapped. At any rate, if there be anything in the differentiation 

 theory, it is in plutonic masses that we shall find evidence in its 

 support. In what follows we shall therefore deal almost exclusively 

 with plutonic phenomena. 



The first area to which we shall refer is the one which has been 

 so exhaustively studied by Professor Brogger (i). Between Lake 

 Mjosen on the N.N.E. of Christiania and the Langesundfjord on the 

 S.S.W. is a tract of country, about 230 kilometres in length and 

 varying from 35 to 70 kilometres in breadth, composed of lower 

 Palaeozoic strata and post-Silurian eruptive rocks. This vast area of 

 about 10,000 square kilometres is separated from the surrounding 

 Archaean rocks by a series of important faults. Sometimes there is 

 one main fault, with a throw of over 1,000 metres, at other times 

 there is a group of smaller faults. In all cases the effect of the 

 faulting has been to depress the rocks within the area above 

 mentioned. This sunken district has been the site of important 

 igneous phenomena, and more than half of it is now covered by 

 igneous rocks, some of which were poured out at the surface, while 

 others were intruded in the form of huge laccolites amongst the 

 sedimentary strata. There is every reason to believe that the 

 sinking of this portion of the earth's surface and the intrusion and 

 extrusion of igneous material are correlated phenomena ; and this 

 district may therefore be quoted in support of the opinion so largely 

 held by the Vienna-school of geologists, that volcanic action is 

 especially characteristic of sinking areas. The fossil-bearing strata 

 attain a thickness of from 700 to 850 metres, and they represent an 

 unbroken succession of Cambrian, Ordovician, and Silurian rocks. A 

 grey or red sandstone, without fossils, probably representing the 

 Old Red Sandstone, overlies the fossiliferous rocks conformably, and 

 attains a thickness of 380 metres. The total thickness of the 



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