PART I. 



30neou0 IRocfcs, 



CHAPTER I. 



THE CONSTITUENTS OF IGNEOUS ROCKS. 



IGNEOUS rocks result from the consolidation of molten masses under 

 various conditions. They may occur as tuffs and agglomerates or as 

 massive rocks. Whether a given mass of molten material shall take the form 

 of a fragmental or massive rock will depend entirely on the conditions of 

 consolidation. If the consolidation be effected beneath the surface a massive 

 rock will, in the vast majority of cases, be produced ; but if, on the other hand, 

 it be erupted at the surface its final condition will depend on the character of 

 the eruption. A violent paroxysmal eruption, such as that of Krakatoa in 

 1883, is accompanied by the escape of enormous quantities of aqueous vapour 

 and other elastic fluids, and the consequent production of fragmental rocks 

 on a gigantic scale. The molten material is distended by the escape of the 

 occluded gases, and a large portion of it is reduced to particles so fine that they 

 may be carried great distances by the wind. A quiet eruption, such as that 

 of Mauna Loa in 1880, is accompanied by the welling-out of large masses of 

 molten material which flow down the slopes of the mountain and finally 

 consolidate as massive rocks. 



In considering the physical, chemical and mineralogical characters of 

 igneous rocks it will be convenient to select the massive rocks as the types to 

 which the fragmental rocks may be referred. Massive rocks may occur as lava 

 flows, veins, dykes, sheets, laccolites (1) and intrusive masses of irregular form. 

 If we examine any extensive series of igneous rocks we recognise at once 

 a great diversity in colour, texture and general appearance. Some contain a 

 number of vesicles or cavities ; others are solid throughout. The latter may 

 appear homogeneous to the unaided eye or they may be seen to consist, either 

 wholly or in part, of distinct minerals. Rocks which appear homogeneous to 

 the unaided eye may often be resolved, by the examination of thin sections under 

 the microscope, into an aggregate of distinct substances. The constituents of 

 massive rocks so far as they can be determined by macroscopic and microscopic 

 examination may be roughly classified as follows : 

 (1) Glass. < 2 > 



(1) G. K. Gilbert. Report on the Geology of the Henry Mountains, U.S. Survey of the 

 Rocky Mountains Region, Washington, 1880, 2nd edition. 



(2) Microfelsite (Rosenbusch) and crypto-crystalline matter are not here referred to as distinct 

 substances. The former is isotropic but differs from glass in the presence of distinct flecks and 

 fibres which are so thickly crowded together as to destroy the clearness of the substance. The 

 latter probably consists of matter in one or more of the above conditions but of so fine a grain as 

 to be incapable of resolution in sections of the usual thickness, and with the magnifying power 

 which is generally employed. In many cases matter which appears crypto-crystalline when 

 examined in thick sections and with low powers may be resolved into a micro-crystalline aggregate, 



