42 JOINTS IN PLUTONIC ROCKS. 



THE JOINT-STRUCTURE OF IGNEOUS EOCKS. 



When igneous rocks cool they all contract, and thus fissures which 

 are called joints appear in them. These joints run through the rock 

 in different directions, according to its composition and the conditions 

 under which it cooled ; and sometimes the same rock presents two or 

 three kinds of joints, or it shows no joints at all. In granite, the 

 prevalent joints run in straight lines which cross each other at some 

 angle ; and in basalt, phonolite, and some other rocks, the joints often 

 form six-sided columns, which may be straight or curved, and vary 

 from an inch or two in diameter up to a width of many feet. Some 

 of the largest may be seen at the old basalt quarries now used for 

 beer-cellars at Nieder-mendig, not far from the Laacher See, on the 

 Rhine. An excellent account of several kinds of joints in volcanic 

 rocks has been given by Professor Bonney, under the names of columnar, 

 tabular, curvi-tabular, and spheroidal structure. 1 There is no doubt 

 that some joints are a consequence of conditions under which the 

 rock cools, but the forms and directions which they assume have 

 always some predisposing cause, usually pressure or strain. The joints 

 in granite could not be accounted for by cooling alone, unless it were 

 supposed that cooling took place from opposite sides of the mass, so 

 that the shrinkage planes formed on one side have intersected those 

 formed on the other side. And it seems likely that jointing is pri- 

 marily a consequence of the development of shrinkage planes in the 

 direction of the predominant arrangement in the rock of its principal 

 mineral constituent. Thus more than half of granite consists of ortho- 

 clase felspar, and if the majority of the felspar crystals have a preva- 

 lent direction, consequent either upon pressure or contraction, then 

 there must have been a tendency for the rock in cooling to behave as 

 though it consisted entirely of felspar, and to divide by joints which 

 correspond more or less with the cleavage planes of orthoclase or with 

 its crystalline faces. And when we bear in mind the circumstance that 

 in granite the minerals have been arranged in at least two directions, 

 it becomes probable that the felspar crystals should have more than 

 one direction, so that a second set of cleavage planes may be produced 

 running through the other minerals associated with the felspar ; and 

 this may be the explanation of the fact, that in most granite quar- 

 ries the joints which correspond with orthoclase cleavage are crossed 

 by others, which at first sight seem to be inconsistent with it, and 

 correspond better with the angular directions of the crystalline faces. 

 In the same way the other kinds of joints might be regarded as 

 consequences of the influence of the rate of cooling upon the mode of 

 arrangement of the predominant mineral forming the rock. Professor 

 Eonney mentions the occurrence of a kind of columnar structure in ice, 

 in haematite iron-ore, in a large quartz vein at Svolvaer in the Lofoden 

 Islands ; in coal where it is in contact with basalt, in volcanic mud 

 beneath basalt at Tideswell Dale in Derbyshire, in the consolidated 

 pelagonite ash of Iceland ; besides finding it in the trachyte of Mont 

 1 Quarterly Journal of the Geological Society, vol. xxxii. p. 140. 



