M 



PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[June 9, 



These two analyses show the important bearing which chemical 

 composition has on jointing ; justifying the conclusion, so far as the 

 limestone of this neighbourhood is concerned, that the perfection of 

 the jointing is in proportion to the purity of the limestone. 



Jointings in other rocks. — That the lithological character of the rock 

 is a matter affecting the nature of the divisional planes known under 

 the name of joints, is in many instances weU shown in rocks which 

 do not appertain to the Carboniferous series, and in which limestones 

 do not occur. 



Among the arenaceous sandstones which constitute the Triassic 

 formation of Cumberland, the effect of a difference in mineral com- 

 position is in some localities well seen. One of these localities is at 

 the quarries of Howrig, near the Curthwaite Station on the Maryport 

 and Carlisle Eailway. Here we have thin-bedded red sandstones, 

 dipping 12°, N. by W. ; and these sandstones are generally intersected 

 by vertical joints, which have a N. and S. course. The joints in 

 each stratum commonly terminate with the bed. The strata of sand- 

 stone are either separated from each other by a thin parting of red 

 shale, or have more argillaceous matter entering into their composi- 

 tion near their bounding places. In either of these cases, the pre- 

 sence of this argillaceous matter produces an effect on the jointings, 

 rendering them less perfect and more indistinct in the more argilla- 

 ceous portions. The result of this change in lithological character 

 is represented in fig. 8. 



Fig. 8. — Jointing in Triassic Sandstone, Howrig, Cumberland. 



Sandstone 



Shale. 



The Silurian rocks of the south of Scotland, in some cases, exhibit 

 the effect of various mineral characters on jointing. In 'ciunnie 

 quarry, in the parish of Troqueer, in the Stewartry of Kirkcudbright, 

 about 2i miles west from the town of Dumfries, this is beautifully 

 represented; the strata here being of greywacke-sandstones and 



