140 LYELL'S ELEMENTS OF GEOLOGY. 



Jointed Structure in Rocks. 



Now these joints are supposed to be analogous to those part- 

 ings which have been already observed to separate volcanic and 

 plutonic rocks into cuboidal and prismatic masses. On a small 

 scale we see clay and starch when dry split into similar shapes, 

 which is often caused by simple contraction, whether the shrink- 

 ing be due to the evaporation of water, or to a change of tem- 

 perature. It is well known that many sandstones and other 

 rocks expand by the application of moderate degrees of heat, 

 and then contract again on cooling ; and there can be no doubt 

 that large portions of the earth's crust have, in the course of 

 past ages, been subjected again and again to very different de- 

 grees of heat and cold. These alternations of temperature have 

 probably contributed largely to the production of joints in 

 rocks. 



In some countries, as in Saxony, where masses of basalt rest 

 on sandstone, the aqueous rock has for the distance of several 

 feet from the point of junction assumed a columnar structure sim- 

 ilar to that of the trap. In like manner some hearthstones, after 

 exposure to the heat of a furnace without being melted, have be- 

 come prismatic. Certain crystals also acquire by the application 

 of heat a new internal arrangement, so as to break in a new di- 

 rection, their external form remaining unaltered. 



Scoresby, when speaking of the icebergs of Spitzbergen, states 

 that " they are full of rents, extending perpendicularly down- 

 wards, and dividing them into innumerable columns." Colonel 

 Jackson, who has lately investigated this subject more attentively, 

 found that the ice on the Neva, at St. Petersburg, at the begin- 

 ning of a thaw, when two feet in thickness, is traversed by rows 

 of very minute air-bubbles extending in straight lines, sometimes 

 a little inflected, from the upper surface of the ice towards the 

 lower, within from two to five inches of which they terminate. 

 '* Other blocks presented these bubbles united, so as to form cy- 

 lindrical canals, a little thicker than a horsehair. Observing still 

 further," he says, " I found blocks in which the process was 

 more advanced, and two, three, or more clefts, struck off in dif- 

 ferent directions from the vertical veins, so that a section perpen- 

 dicular to the vein would represent in miniature the star-formed 

 cracks of timber. Finally, in some pieces, these cracks united 

 from top to bottom of the veins, separating the whole mass into 

 vertical prisms, having a greater or less number of sides. In 

 this state a slight shock was sufficient to detach them ; and the 

 block with its scattered fragments was in all respects the exact 

 miniature resemblance, in crystal, of a Giant's Causeway. The 

 surface was like a tessellated pavement, and the columns rose 

 close, adhering and parallel, from the compact mass of a few 



