748 DYNAMICAL GEOLOGY. 



uean streams ; and the force from generated vapors may work with 

 that from pressure below in producing uplifts of an adjoining region. 



As subterranean waters from superficial or other sources are taken 

 in by the lavas of volcanoes, so may they have been by the igneous 

 material filling fissures, wherever it was accessible to them while it was 

 on the way to the surface. When a fissure intersects sedimentary 

 strata it may encounter subterranean streams of water of great vol- 

 ume, and become thoroughly penetrated by it. Having no open 

 throat to escape by as in a volcano, the moisture, in the form of steam, 

 may saturate the rock, hydrating more or less its minerals, turning 

 pyroxene to chlorite, feldspar to zeolites or chlorite, and making other 

 alterations, besides causiug steam-holes or the so-called amygdaloidal 

 cavities — where the pressure admits of it. So again if limestones 

 are encountered, carbonic acid may be taken in for making, along with 

 the lime of the pyroxene or feldspar, calcite amygdules. Even the 

 mineral oil of carbonaceous shales was taken up in the gaseous state by 

 the trap of the Connecticut valley and deposited in an oxydized con- 

 dition in the amygdaloidal cavities, where it now occurs in black coal- 

 like nodules. The zeolites of amygdaloids, page 777, are other re- 

 sults of this kind of action. 



3. Chronological variations in igneous action As to the kinds 



of igneous rocks, the past and the present are essentially the same. The 

 acidic and basic bear to one another about the same ratio, and have 

 the same composition. Felsyte of the Silurian is identical in mineral 

 composition with the trachyte of modern times, the older doleryte with 

 the most modern, the older dioryte with the modern propylyte and 

 quartz-propylyte, and so on. But in the earlier periods, eruptions 

 were fewer, and mostly through fissures and non-volcanic; in the 

 later, largely volcanic as well as non-volcanic. Volcanoes appear to 

 have been produced in great numbers in the Tertiary, and had then 

 their greatest eruptions. The augmented elevation of the continents 

 during and since the Tertiary may have been one cause of their de- 

 cline in activity. 



In a given region, feldspathic (orthoclase or acidic) igneous ejec- 

 tions have generally preceded dolerytic (basic). On this view their 

 frequent occurrence at the centre of volcanic mountains has usually 

 been explained. But they have sometimes succeeded to dolerytic 

 eruptions, as Lyell observed in Madeira. 



Titles of some of the works and memoirs on Volcanoes and Volcanic phenomena : — 

 G. P. Scrope: On Volcanoes. 8vo, London; 1st edit. 1825, 2d edit. 1862. In both 

 editions attributes fluidity in lavas to aqueo-igneous action, and presents the view that 

 " the originating cause of eruptions and changes of level in the crust is the unequal 

 transmission through it of heat from beneath, owing to variations in the covering sur- 

 faces from the deposition of sediments," " and the abrasion of the land." — Geology of 



