20 SHRINKAGE FISSURES OLDER ROCKS. 



house; and on the outside it weathered similarly 

 and had the aspect of an argillous limestone. On 

 being broken, there were internally nearly parallel, 

 more or less regular, horizontal joints, and two sets 

 of vertical joints, which enable the block to be easily 

 broken into irregular cubical pieces. In the cubical 

 pieces there was a horizontal lining, seemingly due 

 to the packing of the cement in the barrel. 



Joint- lines gradually opening can be studied in an 

 abandoned limestone quarry, where sheets of rock 

 have been uncovered, and left subject, for greater or 

 less periods of time, to atmospheric influences. Here 

 it will be found that in the parts longest exposed, 

 fissures have been produced along the joint-lines; 

 while in the places more recently stripped, the joints 

 remain in their original condition. The fissures are 

 partly due to meteoric abrasion ; but that contraction 

 has also partly formed them, is evident in the places 

 where the joints have cut through layers or nodules of 

 chert, this very hard kind of rock being nearly un- 

 affected by weathering, yet contraction has separated 

 one portion from the other. The connection between 

 contraction and jointing is very perceptible in such 

 rocks as hard conglomerates. In them the surfaces 

 of division will be found to cut through the 

 enclosed pebbles, and each part of any pebble will 

 exactly fit its fellow on the opposite side of the joint, 

 showing conclusively that the jointing could not be 



