374 TRANSACTIONS OF SECTION C. 



infrequently meet with a few extreme types which, occurring in a calcic pro- 

 vince, recall the characters of alkaline rocks, or conversely. Such anomalies have 

 been pointed out by Daly, Whitman Cross, and others. They are found among 

 the later derived types, referable to prolonged or repeated differentiation, and 

 they are to be expected especially where the initial magma was not very strongly 

 characterised as either calcic or alkaline. 



Having regard to the known exposures of igneous rocks over the existing 

 land-surface of the globe, it seems that there is a very decided preponderance of 

 the calcic over the alkaline branch. This, as we shall see, i/3 probably a fact ot 

 real significance, but it is nevertheless noticeable that increasing knowledge tends 

 partly to redress the balance. In our own country, in addition to the Scottish 

 Carboniferous rocks and those probably of Ordovician age in Pembrokeshire, we 

 have the remarkable Lower Palasozoie intrusions of Assynt, in Sutherland, of 

 6trongly alkaline character, as described by Dr. Teall and more recently by Dr. 

 Shand ; while Dr. Flett has recognised alkaline rocks of more than one age in 

 Cornwall and Devon, and Mr. Tyrrell is engaged in studying another interesting 

 province, of Permian age, in Ayrshire. 



That the distinction between the alkaline and the calcic rocks embodies some 

 principle of real and fundamental significance becomes very apparent when we 

 look at the geographical distribution of the two branches. Taking what the 

 German petrographers call the 'younger' igneous rocks, i.e., those belonging to 

 the latest system of igneous activity, we find it possible to map out the active 

 parts of the earth's crust into great continuous regions of alkaline rocks on the 

 one hand and of calcic on the other. An alkaline region comprises numerous 

 petrographical provinces, which may differ notably from one another, but agree 

 in being all of alkaline facies. In like manner a common calcic facies unites 

 other provinces, which collectively make up a continuous calcic region. Concern- 

 ing the igneous rocks of earlier periods our knowledge is less complete, but, so 

 far as it goes, it points to the same general conclusions. 



These considerations enable us to simplify at the outset the problem before 

 us. If we would seek the meaning and origin of petrographical provinces, we 

 must inquire in the first place how igneous rocks as a whole come to group them- 

 selves under two great categories, which, at any one period of igneous activity, 

 are found in separate regions of the earth's crust. The fact that a given district 

 may form part of a calcic province at one period and of an alkaline one at 

 another, precludes the hypothesis that the composition of igneous rocks depends 

 in any degree upon peculiarities inherent from the beginning in the subjacent 

 crust. The same objection applies with scarcely less force to various conflicting 

 suggestions based on an assumed absorption or ' assimilation ' of sedimentary 

 rocks by igneous magmas. Thus Jensen supposes the alkaline rocks to be 

 derived by the assimilation or fusion of alkaline sediments at great depths. Daly 

 propounds the more elaborate, and on a first view paradoxical, theory that alka- 

 line have been derived from calcic magmas as a consequence of the absorption of 

 limestones. These geologists agree in regarding the alkaline rocks as relatively 

 unimportant in their actual development and in some sense abnormal in their 

 origin. For Suess, on the other hand, it is the calcic rocks which owe their 

 distinctive characters to an absorption of sedimentary material, enriching the 

 magma in lime and magnesia. Apart from difficulties of the physical and chemical 

 kind, all such theories fail to satisfy, in that they ignore the separation of the 

 two branches of igneous rocks in different regions of the globe, each of which 

 includes sediments of every kind. What then is the real significance of this 

 regional separation? The obvious way of approaching the question is to inquire 

 first whether the alkaline and calcic regions of the globe present any notable 

 differences of a kind other than petrographical. 



Relation between Tectonic and Petrographical Fades. 



The close connection between igneous activity and displacements of the earth's 

 crust has been traced by Suess, Lossen, Bertrand, de Lapparent, and others, and 

 is a fact sufficiently well recognised. We have here indeed two different ways of 

 relieving unequal stresses in the crust, and it is not surprising that they show a 

 broad general coincidence both in space and in time. We can, however, go 

 farther. Not only the distribution of igneous rocks in general, but the distri- 

 bution of different kinds of rocks is seen to stand in unmistakable relation to the 



