180 PHYSIOGRAPHY 



178). The roof of the intrusion may be broken and lifted up, in- 

 stead of being arched. Lava is sometimes forced in between beds 

 of stratified rock in sheets, and into cracks of the rock. In the 

 latter case it forms dikes. 



Intrusions of lava may give rise to topographic features of 

 importance after erosion has affected the regions where they occur, 

 for the hardened lava (or igneous rock) is often harder than its 

 surroundings. Dikes often give rise to ridges (Fig. 1, PI. XLIII, 

 p. 161). Intruded sheets of lava also, if they have been tilted from 

 a horizontal position, may give rise to ridges, and these ridges may 

 be so high as to be called mountains. The Palisade Ridge of the 

 Hudson (Fig. 115), and most of the mountains of the Connecticut 

 River valley, are examples. Sills and extrusive sheets of lava may 

 also give rise to buttes, mesas (p. 80), rock terraces, etc., indeed, 

 to all the topographic forms which result from the erosion of "rock 

 of unequal hardness (p. 76 et seq.). 



Columnar structure. As lava hardens it sometimes assumes a 

 columnar structure (Fig. 2, PL XLIII, p. 161). The columns are 

 mostly six-sided, as at Giant's Causeway, Ireland, along the Colum- 

 bia River in Washington, and elsewhere. 



Causes of Vulcanism 



The causes of volcanoes lie outside the field of physiography, 

 but it may be stated that the old notion that volcanic vents are 

 connected with a liquid interior has been generally abandoned. 

 Yet it seems clear that the interior of the earth is hot. Deep mines 

 and deep borings of all sorts show that the temperature increases 

 with increasing depth. The rate of increase varies from 1 F. for 

 17 feet in rare cases, to 1 for more than 100 feet. The average 

 rate of increase is commonly stated as about 1 for every 50 to 60 

 feet; but if we take only the records of those deep mines and other 

 borings which seem most reliable, the rate is more nearly 1 for 80 

 feet, down to the greatest depths yet penetrated. It is to be 

 remembered, however, that the deepest excavations are but little 

 more than a mile in depth. 



If the heat increases at the average rate of 1 for each 80 feet, 

 a temperature of 3,000 would be reached at a depth of about 50 



