The Scottish Naturalist. 221 



it is called, a conchoidal fracture. The production of joints in 

 rocks is said to result from the condensation of their mass from 

 an aqueous or igneous expansion. But (I now quote Professor 

 Phillips) the symmetry of their arrangement can only be referred 

 to some kind of crystallic action, and the parallelism of the 

 great joints over large tracts of country seems the effect either 

 of electrical currents controlling that action, or of peculiar, per- 

 haps undulatory, movements affecting large parts of the crust, 

 of the globe. I need hardly quote Fingal's Cave or the 

 Giant's Causeway. 



Professor Faraday divided all bodies into two great classes, 

 those which when placed between the poles of a magnet 

 pointed in a straight line in consequence of their attrac- 

 tion assuming an axial position, and those which are re- 

 pelled by each pole so that they point at right angles to the 

 line joining the poles, that is, in an equatorial direction. 

 This is the relation between magnetism and crystallization, 

 the one class of bodies being magnetic and the other dia- 

 magnetic. A certain line in crystals that tends to place itself 

 parallel to the magnetic axis was called by Faraday the magno- 

 crystallic axis. The Professor also spoke of a force which regu- 

 lates the motion of crystals as magneto-crystallic ; the action is 

 thus described — "The line or axis of the magno-crystallic force 

 tends to place itself parallel or at a tangent to the magnetic 

 curve, or to the line of magnetic force passing through the plane 

 where the crystal is separated." Galvanism is a force that may 

 aid in shaping or modifying forms. Chemical affinity and 

 electricity — two very nearly related forces — as also cohesion, are 

 constantly employed in the formation of these barefaced traps. 

 How much there is then to be learned about these indigenous 

 materials. Subtle forces are now being laid hold of that will ere 

 long lead on to great generalizations. These volcanic rocks are 

 the parents of all other materials on the globe ; the whole of 

 them being for the most part combinations of silicic acid with 

 iron, alumina and magnesia, potash and lime, and soda. It is 

 very wonderful that the vast variety of strata have been formed 

 out of these. 



In the Earn valley below Dupplin many "fortification" 

 agates are found. These have been formed in cavities by the 

 infiltration of silicic acid in a state of solution. Indeed, 

 quartz is silica, and amethyst, cairngorm, jasper, onyx, agate,, 

 catseye, cornelian, bloodstone, opal, chalcedony, flint, cherb, 



