MOUNTING TRANSPARENT ROCK-SECTIONS. . 29 



quarry. The specimens ought to be labelled on the spot, and the 

 most convenient way to do this, is to have with you a quantity of 

 elliptical or circular gummed labels, about three-quarters of an inch 

 in diameter. If the specimen is taken from a Trap Dyke, the 

 thickness or width of the Dyke should be taken, and the particular 

 place from which the specimen was struck noted. 



If the Dyke is a wide one, two, or possibly three sections will 

 be required to show its structure. For in wide Dykes, specimens 

 taken from the middle will be found to be softer, and the crystals 

 larger, than specimens taken from near the side of the same Dyke. 

 Mining men are well acquainted with this, and I may mention 

 it as a fact, that in cutting a mine six feet high by six feet wide 

 through a sixteen fathom Dyke, in the Ayrshire Coal Fields, it 

 cost twenty pounds sterling per fathom, to cut the mine near the 

 sides of the dyke, whereas near the centre it was driven for six pounds 

 per fathom. It would be out of place here to enter into an 

 argument of the causes. — First, why Trap Dykes exist at all, and 

 second, why they are harder and more finely "grained" at the sides 

 than they are in the interior. I may just say, for the benefit of 

 those who have not studied these matters, it is the accepted theory, 

 that Dykes have at one time been molten rock, filling up cracks 

 in the Earth's crust, and that naturally this molten rock cooled 

 quicker at the sides ofthe crack than in the centre, and consequendy 

 the Dyke is more finely crystallised and compact at the sides, than 

 in the interior. 



This, then, I hope, will be a sufficient reason for saying that 

 more sections than one are required to show the Microscopic 

 structure of a wide Trap Dyke, or in fact of almost any Dyke. 



The student who follows these instructions about chipping and 

 cutting, will be able to take home as many specimens in his pocket 

 as he could otherwise do in a large bag, by the old method of simply 

 using the hammer ; and this is no small matter, when one walks 

 perhaps twenty or thirty miles, in a day's hunting after specimens. 



Having got safely home with a variety of specimens, the next 

 thing to be done is to make the discs roughly circular, and to 

 flatten and polish one side. To do this, I use a flat slab of polished 

 sandstone, eighteen inches square by four inches thick, on which 

 I rub the edges of the specimen, using water^ and giving it a slight 



