GLOSSARY OF GEOLOGIC TERMS. 203 
Volcanic neck. A plug of lava that formerly congealed in the pipe of a volcano. 
When the tuffs and lava flows that make up most of a volcano have been washed 
away by erosion the neck may remain as an isolate hill. 
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. 
