310 REPORT On NATIONAL MUSEUM, 1886. 



F.— METHODS OF QUA KEYING AND DEESSING. 

 (1) JOINTS IN ROCKS AND THEIR UTILITY IN QUARRYING. 



All rocks, whatever their origin, are traversed by one or more systems 

 of natural seams or cracks, called joints. These vary greatly, accord- 

 ing to the nature of the rock in which they occur, sometimes being so 

 fine as to be almost imperceptible, or again perfectly distinct and capable 

 of being traced for many yards, or even miles. In stratified rocks (lime- 

 stones, sandstones, schists, etc.), according to Professor Geikie, the 

 joints, " as a rule," run perpendicular, or approximately so, to the planes 

 of bedding, and descend vertically at not very unequal distances, so that 

 the portions of the rock between them, when seen from a distance, ap- 

 pear like so many wall-like masses. An important feature of these 

 joints, as mentioned by this authority, is the direction in which they 

 intersect each other. In general they have two dominant trends, one 

 coincident on the whole with the direction in which the strata are in- 

 clined from the horizon, and the other running transversely at a 

 right angle, or nearly so. The first are called "dip joints" or "end 

 joints" by the quarry men, since they run with the dip or inclination of 

 the rock, while the last are called " strike joints," since they conform in 

 direction to the strike of the rock. These last are also called "back 

 joints." 



In massive rocks like granite and diabase, joints, though prevalent, 

 have not the same regularity of arrangement as in the stratified forma- 

 tions ; nevertheless, most rocks of this class are traversed by two in- 

 tersecting sets, whereby the rock is divided into long, quadrangular, 

 rhomboidal. or even polygonal masses. Frequently, also, there exists 

 a third series of joints running in an approximately horizontal direction, 

 or corresponding more nearly with the bedding in stratified rocks. 

 These are called by quarrymen " bottom joints," since they form the 

 bottom or floor of the quarry. In some instances, as at the Hallowell 

 (Maine) granite quarries, these bottom joints are so pronounced that no 

 artificial means are required to start the rock from its bed after being 

 freed at the sides and ends. 



The cause of these joints has never been fully and satisfactorily ex- 

 plained. By some they are supposed to be due to contraction caused 

 by cooling, and by others it is supposed that they are simply fractures 

 produced by earthquakes. Obviously, the matter can not be discussed 

 here, and the reader is referred to the various text-books on geology. 

 But whatever may have been their origin, their presence is a matter 

 of great importance to quarrymen, and, indeed, the art of quarrying has 

 been well stated by Professor Geikie to consist in taking advantage of 

 these natural planes of division. By their aid large quadrangular blocks 



