186 GUIDEBOOK OF THE WESTERN UNITED STATES. 
Voleanic neck. A plug of lava that congealed in the pipe of a volcano. When 
the tuffs and lava flows that make up most of a volcanic cone have been washed 
away by erosion the neck may remain as an isolated hill. (See fig. 20, p. 97.) 
Volcanic rocks. Igneous rocks erupted at or near the earth’s surface, including 
lavas, tuffs, volcanic ashes, and like material. 
Weathering. The group of processes, such as the chemical action of air and rain 
water and of plants and bacteria and the mechanical action of changes of tem- 
_ perature, whereby rocks on exposure to the weather change in character, decay, 
and finally crumble into soil. (See Pl. XXXVII, p. 140.) 
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