558 ALFRED HARKER 



igneous gneisses, we find that they belong almost exclusively to the 

 calcic branch. A few exceptions there are, and must be. A nephe- 

 line syenite may be intruded in a line of faulting during the time 

 of movement, as in the Langesundsfjord; or it may be crushed and 

 metamorphosed long afterward by stresses with which it has no 

 genetic connection, as at Loch Borolan; but these are isolated and 

 incidental occurrences. The matter is easily brought to the test. 

 In Grubenmann's classification, based solely on chemical composi- 

 tion, the crystalline schists and gneisses of igneous origin are con- 

 tained in six of the twelve groups. The calcic rocks are in Groups 

 I, III, IV, and V, which correspond with granites, diorites, gabbros 

 and peridotites. They include a rich variety of types, and col- 

 lectively make up enormous tracts of the earth's crust. To com- 

 plete his classificatory scheme the author has been able to produce 

 various types of alkaline rocks, which scantily furnish forth Groups 

 VI and VII, but most of them are little more than petrographical 

 curiosities. In respect of the total bulk of all known occurrences, 

 these alkaline crystalline schists as a whole are quite insignificant 

 as compared with any single type in the calcic division. 



The striking disparity here noted is only one consideration 

 among others which points to a peculiar distribution of alkaline 

 and calcic igneous rocks in relation to crustal stresses. If anyone 

 seriously believes that such things are matters of chance coincidence, 

 there is no more to be said. It is to be hoped rather that chemists, 

 as well as geologists, will recognize here a real significance, and will 

 lend their help in the attempt to explain the facts, not to explain 

 them away. 



