FAULTS, JOINTS AND CLEAVAGE. 15 



the fact that the partings are locally arranged on a definite system. In 

 slaty cleavage the cleavage planes are substantially parallel and very 

 close together ; in flags of the slaty class the intervals between cleavage 

 pianos are greater ; in schists the partings range through small angles, and 

 in these last rocks there are frequently two sets of partings, each cleavage 

 making a small angle with others of the same set, but a large angle with 

 those of the other set. Where the rock is divided by cracks these are 

 often parallel and spaced with a considerable approach to uniformity. 

 Sometimes they occur at a fraction of an inch from one another, while in 

 other instances they are rods apart. In still other cases there are two 

 systems of such cracks crossing one another at right angles, or at angles 

 which approach to right angles. Not infrequently such a double system 

 of fissures is accompanied by a second of like character, at right angles to 

 it, dividing the rock into polyhedral fragments of greater or less size. 



Slaty cleavage is at present regarded by most geologists as due to a 

 pressure acting in a direction perpendicular to the planes of cleavage, 

 and this opinion is supposed to be well supported by experiments. 

 Indications are not wanting, however, that many observers are ill satisfied 

 with this explanation. Less attention has been paid to jointing, concern- 

 ing which there is no consensus of opinion. By some it is considered as 

 due to tensile stresses, while others insist on its intimate association with 

 cleavage. Jointing is also often treated as distinct from faulting and as 

 being unaccompanied by any relative movement of the joint walls. No 

 systematic attempt appears to have been made to elucidate these various 

 structures, which are generally recognized, however, as at least sharing a 

 dynamic origin. Even the experiments on cleavage seem to me not to 

 have been studied with as much care as they deserve. 



Scope of the Inquiry. — Orogeny can never be satisfactorily discussed 

 until the dynamic significance of cleavages and cracks is clear. A neces- 

 sary step toward this end consists in the elucidation of those areas, great 

 or small, throughout which the phenomena are uniform; for, however 

 complex the conditions may be in any body of rock, they may be con- 

 sidered as uniform over a sufficiently small fraction of the whole mass. 



Even this seemingly modest step cannot be completed in the present 

 state of science. In the mechanics of artificial structures and machinery 

 it is sufficient to discuss very small deformations, for such only are ad- 

 missible. In geology this is wholly insufficient, the strains frequently 

 being of enormous amount ; so great indeed that laboratory experiments 

 hardly aid one to conceive that they are possible. Yet there is no doubt 

 among geologists that pebbles, even of quartzite, in conglomerates are 

 not infrequently elongated by pressure to double their original length 

 without rupture. Thus in geological mechanics it is absolutely essential 



IV— Bum,. Geol. Son. Am., Vol. 4 18!)2. 



