216 STRUCTURAL GEOLOGY. 



extensive sheets. In the one the erupted matters ac- 

 cumulate about the vent as a cone ; in the other they 

 form great lava-fields,, or else may be forced between the 

 strata and never come to the surface at all. In the one 

 the force of ejection is probably the elastic force of vapors, 

 as explained under volcanoes ; in the other the force is 

 more obscure, but probably of the same nature as that 

 which forms mountains. The two kinds may be called 

 crater-eruptions and fissure-eruptions. At present only 

 the former kind seems to exist ; and therefore in Part I, 

 while treating of causes now in operation, we treated only 

 of this mode. But in studying erupted materials of all 

 periods, it is plain that by far the larger quantity have 

 come up in the second way. 



Modes of Occurrence. Leaving out of view those 

 modes of occurrence already described under volcanoes, 

 viz., chimney-cones with radiating dikes and lava-streams, 

 the principal modes of occurrence of eruptive rocks are : 

 1. Dikes. 2. Overflow-sheets. 3. Intercalary beds. 



1. Dikes. Dikes are vertical sheets filling great fis- 

 sures in stratified or other igneous rocks. They are the 

 most common of all modes of occurrence of eruptives and 

 intrusives. In all mountain-regions they are found in 

 great numbers. In width they vary from a few feet to 

 hundreds of feet, and may often be traced outcropping 

 over the surface fifty to one hundred miles. But since 

 rocks are usually covered with soil, they are not always 

 visible at once, but must be looked for wherever the rock 

 is exposed, especially in stream-beds. 



It is evident that fused matter coming to the surface 

 must overflow, and therefore dikes thus outcropping on 

 the surface are either the exposed roots of former over- 

 flows which have been removed by erosion, or else are the 

 fillings of fissures which never reached the surface at all 

 (Fig. 121, V). In either case, an outcropping dike is the 

 sign of gisat erosion. If, therefore, the dike is harder 



