Igneous Rocks. 39 



to be igneous by their crystalline, slaggy, scoriaceous, 

 vesicular, or columnar structures, and also by the effects 

 they have produced on the strata 

 with which they are associated. 

 Shales, sandstones, &c., are often 

 hardened, bleached, and even 

 vitrified at the points of junc- 

 tion with greenstone, basaltic, i. Dyke with veins. 

 and felspathic dykes, or old lava 2 - Overflow of basaltic lava. 

 j // n> j XT i j 3 - Altered strata at junction, 



beds (fag. 9), and the same kind 4. Unaltered sandstone and 



of alteration takes place on a shale - 



greater scale when large masses of igneous rocks have 



been intruded among the strata. 



Then by comparing volcanic rocks of old date with 

 those of modern origin, we are able to decide with 

 perfect truth, that rocks which were melted long before 

 the human race appeared upon the world are yet of 

 truly igneous origin. 



Changes of a more general character are especially 

 marked in cases where granite, syenite, felspar and 

 other porphyries and their allies, are associated with 

 stratified deposits. Their igneous affinities are known 

 by their crystalline structure, their modes of occurrence, 

 and the effects they produce on the strata. Granite is 

 composed of crystals of quartz, felspar, and mica ; and 

 syenite, according to old nomenclatures, of quartz, 

 felspar, and hornblende. They often send veins or dykes 

 into stratified rocks with which they are in contact, 

 as in figs. 10 and 11, and frequently all along the 

 line of junction, and often at great distances from it, 

 alterations of the strata of an extreme character (meta- 

 morphism) are common. One marked distinction be- 

 tween granitic and volcanic and ordinary trap rocks is, 

 that though injected veins of granite are common, granitic 



