236 



WESTERN 



Volcanic neck- A plug of lava that formerly congealed in the pipe of a volcauo. 

 WTien the tuffs and lava flows that make up most of a volcano have been washed 



away by erosion the neck may remain as an isolated hill. 



rocks 



lavas, tuffs, volcanic ashes, and like material. 



surface, including 



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. 



