THE COMPOSITION OF THE EARTH 



53 



as an abrupt, steep-sided hill. These elevations, known as vol- 

 canic necks or plugs (Figs. 32 and 33), range in diameter from 

 a few yards to a mile or more. They may be regarded as mon- 

 uments, marking the sites of volcanoes which died ages ago. 

 Volcanic necks are known at various points in the West, espe- 

 cially New Mexico, in Scotland, and in many other places. No 

 matter how resistant their rocks, volcanic necks are them- 

 selves finally destroyed as topographic features, leaving as 

 perhaps the only record 

 of the ancient volcanoes 

 the igneous rock occupy- 

 ing the old tubes lead- 

 ing to unknown depths 

 below. There are many 

 examples of this stage in 

 the West. 



Columnar structure. 

 The cracking of fine mud 

 as it contracts on drying 

 is a familiar phenomenon. 

 In a similar way, some 

 lava cracks on cooling, 

 sometimes forming regu- 

 lar columns (Fig. 34). 

 These are six-sided in 

 many cases, and stand at 

 right angles to the cool- 

 ing surfaces. In horizontal sills and lava flows, therefore, the 

 columns are vertical, while in vertical dikes they are horizon- 

 tal. They occur, among other places, in the Palisades of the 

 Hudson, and in Mount Holyoke, Massachusetts. The cracks 

 which separate the columns are joints. Joints are not pecu- 

 liar to igneous rocks. They affect rocks of all kinds, dividing 

 them into blocks of various sizes and shapes. 



FIG. 34. The Devil's Post Pile in the 

 Sierra Nevada Mountains. Basalt which 

 has split into columns. (Nat. Geog. 

 Mag.) 



