UNSTRATIFIED OR IGNEOUS ROCKS. 



217 



than the country-rock through which it breaks,, it will 

 stand above the surface and look like a low, ruined wall 



"T=T 



FIG. 124. Dikes. 



(Fig. 124, a). If, on the contrary, the igneous rock yield 

 more easily to erosion than the country-rock, then it may 

 be traced as a shallow, half-filled ditch (Fig. 124, b). 



Effect of Dikes on Stratified Rocks. On both 

 sides of a dike the bounding walls of stratified rock are 

 always changed by the intense heat of the fused matter. 

 Sandstones are changed into a rock resembling gneiss 

 (page 225), clays are baked into porcelain jaspers, lime- 

 stones are changed into crystalline marbles, coal-seams 

 into anthracite and sometimes into coke. In all cases the 

 fossils, if any, are more or less completely destroyed. 

 These metamorphic changes usually extend only -a few 

 feet or yards from the place of contact. 



2. Overflows. This is the next most common form of 

 occurrence. The liquid matter has come up through great 



illlllllllllllliUUIllllllllIll 



FIG. 125. Lava sheets. 



fissures, such as are made by crust-movements, and spread 

 on the surface as extensive sheets. Often sheet after sheet 



