Igneous Rocks. 



21 



ficial lava, formed of the silica and alumina of the iron 

 ore and its flux of lime, melted together and still re- 

 taining a percentage of iron. Ancient lavas, such as 

 those of Snowdon, of Lower Silurian age, often still 

 possess a slaggy and ribboned structure. Further, 

 igneous rocks are apt to alter any strata through which 

 they are ejected or over which they flow. Accordingly, 

 in rocks of all ages, and of various composition, fels- 

 pathic, doloritic (hornblende and felspar), dioritic 



FIG. 4. 



(augite and felspar), and various others, as in fig. 4, 

 we frequently find veins (2) that have been injected 

 among the strata, from dykes, as they are termed (1), 

 rising vertically or nearly vertically through the beds 

 from the end of which sometimes an overflow of lava 

 (3) proceeded, that may or may not be columnar. In 

 such cases the stratified rocks are apt to be altered for a 

 few inches or even for several feet at their junction 

 with the igneous rocks. If shales, they may be hard- 

 ened or baked into a kind of porcellanic substance ; if 

 sandstones, turned into quartz-rock, something like the 

 sandstone floor of an iron-furnace that has long been 

 exposed to intense heat. Occasionally the strata have 

 been actually softened by heat, and a semi-crystalline 

 structure has been developed. 



From these and many other circumstances, a skilled 

 geologist finds no difficulty in deciding that such and 

 such rocks are of igneous origin, or have been melted 



