REPORT OF TEE CHIEF ASTRONOMER 699 



SESSIONAL PAPER No. 25a 



gives a temperature of 1200° C. at the depth of 40 kilometres (25 miles). Since 

 both conductivity and diffusivity for heat are notably lowered by increase of 

 temperature, it is quite possible that the temperature gradient steepens with 

 depth. In that case a temperature of 1200° C. may reign at an average depth 

 of less than 40 kilometres. 



On the other hand, it is clear that the temperature gradient is, in part, the 

 result of radioactivity in the rocks of the earth's crust, and that this subatomic 

 energy is one cause of magmatic heat. The relative importance of the primitive 

 heat and of that due to radioactivity is at present beyond even a guess. The 

 subject is full of difficulties and geologists must wait for the physicists to make 

 the balance true. Meanwhile, the analogy of the sun and the other planets 

 can be trusted to enforce the belief in primitive heat. 



Composition of the Substratum — the General Earth-magma. 



In several papers the writer has expressed the opinion that the substratum, 

 which before injection is at least potentially fluid, and after injection is really 

 fluid, is of basaltic or gabbroid composition. The basaltic substratum is con- 

 ceived as the heat-bringer in all igneous activities since the later pre-Cambrian 

 periods. Since the time when the Keewatin greenstones were extruded, if not 

 from a still earlier period, the only primary magma has been the basaltic. All 

 other magmas are conceived to be either differentiates from basalt; or, secondly, 

 direct products of the solution of the over-lying crust; or, thirdly, differentiates 

 of those syntectic* products. The writer has found that these views are, in 

 part, as old as Bernhard Cotta's ' Geologische Fragen,' published in 1858, though 

 Cotta did not, and could not in his day, appreciate the importance of magmatic 

 differentiation. 



Cotta's main idea, which has been independently reached by the present 

 writer through a study of eruptive fields and of the more modern geological 

 literature, is basal to the following considerations on the theory of igneous rocks. 

 The grounds for belief that basalt has been the universal magma since a pre- 

 Cambrian epoch should be briefly restated. 



1. All of the first-class lava floods of the world, from the late pre-Cambrian 

 to the Pleistocene, are composed of basalt remarkably uniform in chemical 

 character. These floods were erupted through fissures. The high fluidity of the 

 basalts, in the act of eruption, is shown by the great area covered by even the 

 thinner flows. Yet in most cases this evidently superheated lava has not dis- 

 solved any appreciable amount of the schists, gneisses, granites, or sediments 

 through which the fissures were opened. The basalt of fissure eruptions is not 

 a syntectic. We can only conclude that the lava channels within the generally 

 acid crust were always narrow and that the basalt was extruded rapidly. Such 

 is the orthodox view. The superheat indicated by the extrusion and form of the 

 flows is an a priori ground for believing that these basalts are not the product 



* " Syntectic " is Loewinson-Lessing's useful name for the mixture of rocks due to 

 their melting together, thus forming a mutual solution. 



