VOLCANOES AND IGNEOUS INTRUSIONS 



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They vary in width from a fraction of an inch to several hundred feet. 

 Their length may be considerable ; one in the north of England runs 

 from the coast inland for 

 about 100 miles, and a length 

 of 5 to 20 miles is not un- 

 common. In Scotland a 

 series of lava dikes run par- 

 allel to each other for a dis- 

 tance of from 20 to 30 miles, 

 while on the coast of New 

 England and in many other 

 parts of North America they 

 are very common. When 

 the surrounding rocks decay 

 more easily than the dike 

 rocks, the latter project 

 above the surface of the 

 ground like walls (Fig. 320) 

 and are sometimes used in 

 Scotland as inclosures. Near 

 Spanish Peaks, Colorado, a 

 dike stands as a great wall 

 100 feet high. Occasionally the dike rock weathers more readily 

 than that which it cuts (Fig. 321), in which case the position of 

 the dike may be indicated by a trench-like hollow. When the 

 dike and the surrounding rocks are about equally resistant, no 



topographic features 



result. 



The texture of the 

 rocks of dikes de- 

 pends upon a num- 

 ber of conditions : 

 (1) if the fissure 

 through which the 

 lava was forced was 

 narrow, the rock of 

 the dike is either 

 glassy or finely crystalline; (2) if, however, the fissure was wide, 

 the dike rock may be coarsely crystalline, with narrow margins of 

 less crystalline or glassy rock. 



Fig. 320. — Dikes cutting flat-lying Eocene 

 strata. West Spanish Peak, Colorado. 

 (U. S. Geol. Surv.) 





Fig. 321. — Diagram showing the effect of weathering 

 upon two dikes (shown by horizontal lines), one of which 

 is more resistant than the surrounding rock and the other 

 less resistant. 



