358 



SCIENCE 



[N. S. Vol. XXXIV. No. 873 



The "spilitic" suite of Dewey and Flett 

 is made to embrace a somewhat miscel- 

 laneous collection of types, and any close 

 genetic relationship among them can 

 scarcely be considered as proved. It is 

 perhaps permissible to suggest that, e. g., 

 the quartz-diabases are, here as in Scot- 

 land, quite distinct in their affinities from 

 the types rich in soda. These latter, con- 

 stituting the bulk of the proposed suite, 

 would seem to belong quite naturally to 

 the alkaline branch, the question of the 

 magmatie or solf ataric origin of the albite 

 being in this connection immaterial. 



A given petrographical province is 

 either of calcic or of alkaline facies, 

 typical members of the two branches not 

 being found together. The apparent ex- 

 ceptions are, I think, not such as to modify 

 very seriously the general rule. Mr. 

 Thomas, in describing an interesting suite 

 of rocks from western Pembrokeshire, 

 recognizes the alkaline affinities of most of 

 them, but assigns some of the more basic 

 types to the opposite branch. In a very 

 varied assemblage we not infrequently 

 meet with a few extreme types which, oc- 

 curring in a calcic province, 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 differ- 

 entiation, and they are to be expected espe- 

 cially where the initial magma was not 

 very strongly characterized 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, is probably a fact of real significance, 

 but it is nevertheless noticeable that in- 

 creasing knowledge tends partly to redress 



the balance. In our own country, in addi- 

 tion to the Scottish Carboniferous rocks 

 and those probably of Ordovician age in 

 Pembrokeshire, we have the remarkable 

 Lower Paleozoic intrusions of Assynt, in 

 Sutherland, of strongly alkaline character, 

 as described by Dr. Teall and more re- 

 cently by Dr. Shand; while Dr. Flett has 

 recognized alkaline rocks of more than one 

 age in Cornwall and Devon, and Mr. 

 Tyrrell is engaged in studying another in- 

 teresting province, of Permian Age, in 

 Ayrshire. 



That the distinction between the alka- 

 line and the calcic rocks embodies some 

 principle of real and fundamental signifi- 

 cance 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 pos- 

 sible 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 prov- 

 inces, 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 col- 

 lectively make up a continuous calcic re- 

 gion. Concerning the igneous rocks of 

 earlier periods our knowledge is less com- 

 plete, but, so far as it goes, it points to the 

 same general conclusions. 



These considerations enable us to sim- 

 plify at the outset the problem before us. 

 If we would seek the meaning and origin 

 of petrographical provinces, we must in- 

 quire in the first place how igneous rocks, 

 as a whole, come to group themselves under 

 two great categories, Avhich, at any one 

 period of igneous activity, are found in 

 separate regions of the earth's crust. The 



