50 



PHYSICAL GEOLOGY 



a few inches to two or three hundred feet, and in exceptional 

 cases have a length of scores of miles. In many cases dike rock 

 is more resistant than the adjacent country rock, and hence 



many dikes form 

 low, narrow ridges 

 (Fig. 27). If softer 

 than the rock it 

 penetrates, the line 

 of outcrop of the 

 dike rock becomes 

 a depression. 



Lava intruded be- 

 tween rock layers in 

 wedge-shaped sheets 

 forms sills (Fig. 26). 

 Sheets of lava ex- 

 truded upon the 

 surface may later 

 be 'covered by other 

 rocks. Such sheets 

 then have the po- 

 sition of sills, though 

 of different origin. 

 In working out the 

 geological history of a region, it is sometimes important to 

 determine whether a given lava sheet which lies between sed- 

 imentary beds is intrusive or extrusive. If the bottom of 

 the bed resting on the lava sheet has been baked by the hot 

 lava, which origin may be inferred? If the top of the lava 

 sheet is glassy and has a vesicular texture? If tongues of 

 igneous rock extend from the lava sheet into the overlying 

 rock? By these and other observations, the problem may 

 usually be solved. 



Sills merge into dome-shaped intrusions. If these merely 

 arched the overlying beds, they are called laccoliths (Figs. 28 

 and 29). The Henry Mountains of Utah are notable ex- 



FIG. 27. Porphyry dike cutting tuff. South- 

 western Colorado. (Howe, U.S. Geol. Surv.) 





