ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 121 



developed columnar structure.* A similar structure, accom- 

 panied by partial coking, is often produced in coal-beds whicb 

 are traversed by whin-stone dykes, as may be seen in many parts 

 of Northumberland. Specimens of this kind of change are 

 exhibited in the Museum of the Royal Geological Society of 

 Cornwall, at Penzance. 



An example of columnar jointing produced by the action of 

 some little-known crystallizing force is the following : — 



"In the gypsum quarries of Chaumont, Montmartre, two 

 beds of crystalline granular gypsum, each 6 or 8 feet thick, occur 

 interstratified with fresh-water marls and limestones, each being 

 effected by a prismatic jointing which does not appear in the 

 clay. The prisms are mostly pretty regularly triangular and 

 hexagonal, and seem to have been produced by the intersection 

 of three sets of vertical equidistant planes crossing each other at 

 angles of 60°."f 



A somewhat similar phenomenon is often observable in the 

 crystallization superinduced in stalactitic masses of carbonate of 

 lime. These at first are usually quite devoid of crystalline 

 structure, but after a time are found to be traversed by planes 

 of cleavage, exactly agreeing with the cleavage-planes of ordinary 

 calcite. 



Joint-structures in eruptive rocks. The remarkably well- 

 developed joint-structure of the granites of the West of England 

 is a matter of general observation. Almost every headland in 

 the St. Just district affords notable examples of prismatic or 

 cuboidal jointing, while tabular masses, which by weathering 

 have been converted into tors of most fantastic appearance, occur 

 on most of the highest hills.* The nature of the jointing in a 

 granite quarry is of the very highest importance to the quarry- 

 man. If the joints are too near each other the stone is for most 

 purposes valueless ; if too far apart the labour of working is 

 correspondingly increased ; if they run obliquely much labour is 

 necessary to prepare the stone for ordinary uses. With horizontal 

 "beds," vertical "backs," (joints running right and left and 

 facing the quarryman), not too far apart, and conveniently 



*See Rutley, Study of Rocks, p. 258, note. 



fjukes and Geikie, Manual of Geology, 2nd ed., p. 181. 



