264 



DYNAMICAL GEOLOGY. 



can be opened about a point on its surface by equable contraction is three, 

 223. ^^^^ hence this number is the easiest to make ; and since three 

 such lines symmetrically placed make angles with one another 

 of 120° (Fig. 223), the hexagonal prism, more or less regular, is 

 the most common form of the •' basaltic " column. 



In the case of large dikes between walls of rock, the set of 

 divisional planes which is nearly or quite vertical is generally more strongly 

 developed than the others, and this occasions a laminated structure in that 



direction looking like 



224 



upturned bedding. 

 This structure is com- 

 mon in the trap of the 

 Triassic of the Con- 

 necticut valley ; and at 

 the same time joints 

 transverse to the ap- 

 parent lamination also 

 occur. In many of the 

 nearly vertical fronts, 

 these two courses of 

 joints are predominant. 

 {b) Contraction from 

 cooling lulien the heat is 

 short of fusion often 

 produces columnar 

 fractures in fine- 

 grained rocks. Sand- 

 stones are thus made 

 columnar by contact 

 with melted rocks. 

 Part of the eifect is 

 due to drying. 



4. Expansion and 

 contraction in the pro- 

 cess of solidification 

 and fusion. — Since the 

 glass state of a mineral 

 or rock is a conse- 

 quence of rapid cool- 

 ing from fusion, and 

 the stone state is the 

 result of slow cooling, 

 glass will become stone 

 if melted and very 

 slowly cooled. In passing from the liquid to the glass state, in the case of 



Obsidian columns, Yellowstone Park. Iddings. 



