NON-YOLCANIC IGNEOUS ERUPTIONS. 



703 



pages 86-89, except the scoriaceous and glassy kinds. Dolerite, 

 basalt, diorite, and porphyry are the most common. They are 

 often cellular, owing to inflations by steam or other vapors ; but 

 the cellules have generally a smooth or even surface within, and 

 are not ragged like those of lavas, — a fact due to their having been 

 under pressure when formed. When cellular, the rocks are said to 

 be amygdaloidal, and are often called amygdaloids, — the cavities being 



Fig. 975. 



Basaltic columns, 



of Ulawana, New South Wales. 



occupied usually by zeolites in nodules which are sometimes almond- 

 shaped. 



The fissures were formed by a fracturing of the earth's crust down 

 to some region of liquid rock, if not to the earth's liquid interior. 

 They have thus the same origin as volcanoes, — but with this differ- 

 ence : that the fissures were not so large as to remain open vents. 



In many cases these fissures have been made through the sub- 

 sidence of an area of depression, which was continued until the in- 

 creasing tension on the lower side of this inverted and subsiding 

 arch of rock finally caused fractures opening from below upwards, 

 that gave exit to the liquid rock. The origin of the dikes in the 

 Connecticut Eiver valley and of those inthe Lake Superior region 

 has been thus explained on pages 432 and 195. 



The great numbers and very wide distribution of such dikes over the globe, 

 taken in connection with the distribution of volcanoes, and regions of ineta- 

 morphism, leave little room for doubt that the interior of the earth is in a 

 liquid state, notwithstanding the results of some mathematical calculations. 



