22 THE MICROSCOPE. 



ness 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 of the crack that in the center, and consequently 

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

 in the interior. 



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

 sections than one are required to show the microscopic structure of 

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



The student who follows these instructions about chip- 

 ping 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 sand- 

 stone, eighteen inches square by four inches thick, on which I rub 

 the edges of the specimen, using water, and giving it a slight turn at 

 every rub; a very little practice will enable anyone to make the discs 

 almost circular. But what is chiefly to be aimed at in making them 

 circular, is to get a smooth edge, as a disc having a perfectly smooth 

 edge will not break so readily in the subsequent process, as a rough- 



