REPORT OF TEE CHIEF ASTRONOMER xix 



SESSIONAL PAPER No. 25a 



Page. 

 Average ratio of calcium to magnesium in the limestones of the 



different periods. , 664 



Origin of certain iron ores, cherts, and jaspers 669 



Origin of the petroleum and natural gas emanating from pre- 



Cambrian sediments 669 



Direct evidence of the chemical precipitation of the carbonate rocks 



in the Priest river and Belt-Cambrian terranes 670 



Summary 672 



Premises 672 



Conclusions 673 



CHAPTER XXIV. 



Introduction to the theory of igneous rocks 677 



Classification of the igneous rocks 677 



Average compositions of leading types. . - 679 



Average specific gravities of certain types 696 



Source of magmatic heat 696 



Composition of the substratum; the general earth-magma 699 



* Primary acid shell of the earth 702 



Abyssal injection of mag-ma 705 



Origin of volcanic action 707 



CHAPTER XXV. 



Classification of igneous intrusive bodies 715 



Introduction 715 



Principles of classification 715 



Injected bodies 716 



Dike 716 



. Intrusive sheet 717 



Laccolith 717 



Phacolith 718 



Bysmalith . . . 719 



Volcanic neck t 719 



Chonolith 719 



Ethmolith 720 



Subjacent bodies 720 



Boss 720 



Stock 721 



Batholith. . 721 



Proposed classification ; 722 



