VOLCANIC OR ERUPTIVE ROCKS. 209 



great fissures formed by crust-movements and spread out as extensive 

 sheets ; in the other they have come up through chimneys and run off 

 as streams. The one may be called fissure- eruption, the other crater- 

 eruption or volcanoes proper. The one gives rise to extensive lava- 

 fields, the other to lava-cones. The force of eruption in the one case 

 is probably either the same as that which makes mountains — i. e., the 

 lava is squeezed out by interior contraction of the earth, or else, in 

 some cases it may be hydrostatic — i. e., a welling out of a lighter liquid 

 by the sinking of a heavier crust within it ; the force, in the other case, 

 is evidently the pressure of elastic gases, especially steam, as already 

 explained (p. 97). We owe this distinction mainly to Richthofen, but 

 it is now universally adopted in this country and quite generally in 

 Europe. According to Richthofen, primary eruptions come always 

 through great fissures and only at great intervals of time; afterward, 

 surface-waters percolating through these fissure-erupted masses, still 

 liquid within, give rise to secondary eruptions through craters. We 

 have no examples of fissure-eruptions taking place at the present time, 

 and therefore, in treating of igneous agencies in Part I, we spoke only 

 of crater-eruptions. But it is impossible to explain the mode of occur- 

 rence of eruptives in the older rocks unless we admit eruptions in early 

 geological times of a different kind from those occurring now in vol- 

 canoes. 



Modes of Occurrence. — What we say under this head refers mainly 

 to fissure-eruptions. True eruptive rocks occur : 1. As extensive 

 vertical sheets filling great fissures which by subsequent erosion out- 

 crop as great dikes, or else filling smaller radiating volcanic fissures 

 as radiating volcanic dikes ; 2. As sheets between the strata (interca- 

 lary beds) as if forced between the separated strata, or else outpoured 

 on the bed of sea or lake, and again covered with sediments ; 3. Out- 

 poured on the land-surface as sheets or streams ; and, 4. In the form of 

 great dome-like masses on the surface or between the strata. 



Dikes. — The fillings of great fissures outcropping on land-surfaces 

 are called dikes. They are very abundant in all the older stratified 

 rocks, especially in mountain-re- 

 gions. They vary in thickness 

 from a few inches to fifty or one 

 hundred feet ; they may be traced 

 over the country sometimes for 

 many miles, even fifty or one 

 hundred, and extend downward 

 to great but unknown depths. 

 Such dikes, outcropping over the face of the country, may be the ex- 

 posed roots of ancient overflows Vhich have been removed by subse- 

 quent erosion (Fig. 180, b); or they may be fillings of fissures which 

 14 



I a 





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^^Jjgm**:^:^m 



- V'V*^r 



- - - ~-i8— ■>-__- ■- - =- y 



— _. — : 



— z -f=M- - - - - = 



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Fig. 184.— Dikes. 



