328 



PHYSICAL GEOLOGY 



Fig. 326. — Section A and map B of a stock 

 or boss. The granite intrusion being more re- 

 sistant than the enclosing rock forms a hill. 



that in which they were intruded, they often form knob-like elevations 

 and are consequently often called bosses. Stocks resemble volcanic 



necks (p. 3 16), but are usually 

 larger; the term neck, more- 

 over, is employed only when 

 there is evidence that it 

 represents the chimney of a 

 volcano. 



Batholiths (Greek, bathos, 



depth, and lithos, stone) are 



great irregular masses of 



igneous rock which stopped 



in their rise many feet from 



the surface of the earth, but 



have since been exposed by 



erosion. They are often 



many hundreds of square 



miles in area and may be 



considered as merely very 



large and irregular stocks. 



In the aggregate these bodies cover many thousands of square 



miles, and although less striking are much more important than 



volcanoes. 



Some Effects of Intrusions. — The rock with which a molten 

 magma comes in contact is more or less changed ; the larger and holttr 

 the intrusions the greater being the effect. This phenomenon will 

 be discussed under metamorphism (p. 341). It is believed that some 

 of the explosions which have taken place on or near volcanoes were 

 due to the presence of molten rock at a short distance below the 

 surface. Since igneous rocks are usually harder than those into which 

 they are intruded, they are often left in relief as buttes (p. 106) and 

 bosses, as the land is reduced by erosion. 



Since igneous rocks are composed of minerals which differ in com- 

 position and often in color and therefore expand and contract differ- 

 ently when heated and cooled, we find in desert and tropical regions 

 that granites and other igneous rocks exfoliate (p. 32) under the 

 Influence of diurnal temperature changes (p. 31), producing sphe- 

 roidal bowlders which are often poised on rounded surfaces. These 

 rocks resemble glacial bowlders, and the smooth surfaces on which 

 they rest, roches moutonnees (Fig. 142, p. 157). 



