146 TOWN GEOLOGY. [vi. 



Now what has made this change in the rock ? We 

 do not exactly know. One thing is clear, that the 

 particles of the now solid rock have actually moved on 

 themselves. And this is proved by a very curious 

 fact which the reader, if he geologises about slate 

 quarries much, may see with his own eyes. The fossils 

 in the slate are often distorted into quaint shapes, 

 pulled out long if they lie along the plane of cleavage, 

 or squeezed together, or doubled down on both sides, 

 if they lie across the plane. So that some force has 

 been at work which could actually change the shape of 

 hard shells, very slowly, no doubt, else it would have 

 snapped and crumbled them. 



If I am asked what that force was, I do not know. 

 I should advise young geologists to read what Sir 

 Henry de la Beche has said on it in his admirable 

 "Geological Observer/' pp. 706-725. He will find 

 there, too, some remarks on that equally mysterious 

 phenomena of jointing, which you may see in almost 

 all the older rocks ; it is common in limestones. All 

 we can say is, that some force has gone on, or may be 

 even now going on, in the more ancient rocks, which 

 is similar to that which produces single crystals ; and 

 similar, too, to that which produced the jointed crystals 

 of basalt, i.e. lava, at the Giant's Causeway, in Ireland, 

 and Staffa, in the Hebrides. Two philosophers Mr. 

 Eobert Were Fox and Mr. Eobert Hunt are of opinion 

 that the force which has determined the cleavage of 

 slates may be that of the electric currents, which (as is 

 well known) run through the crust of the earth. Mr. 

 Sharpe, I believe, attributes the cleavage to the mere 

 mechanical pressure of enormous weights of rock, 

 especially where crushed by earthquakes. Professor 



