W. 0. Crosbi/ — Absence of Joint- Structure at Great Bept/is. 419 



subject that contraction-joints have their best development at the 

 surface, gradually dying out at considerable depths, but constantly 

 growing downwards as the surface is lowered by denudation. 



This argument for the essentially superficial nature of joint- 

 structure, derived from considerations touching upon its origin, is, 

 in itself, perhaps, too speculative to carry conviction to many minds ; 

 but I feel that this criticism will not lie with equal force against the 

 ai'gument based upon the relations of the jointing of the rocks to the 

 forms of eruptive masses, which I now proceed to present. 



There are few, if any, geologists who will question the truth 

 of the proposition that, in general, other things being equal, the 

 coarseness of the crystallization of eruptive rocks is pi'oportional to 

 the depth below the surface at which they have cooled and solidified ; 

 and consequently that we see coarsely crystalline eruptives exposed 

 on the surface now only as the result of extensive erosion or fault- 

 ing ; so that, when these rocks are exposed by erosion, as in most 

 cases, the mean size of the crystals becomes a rough measure of the 

 amount of denudation which the region has suffered, and of the age 

 of the intrusives. 



It is also a fact, well known to most field-geologists at least, that 

 the regularity of the outlines or boundaries of an intrusive mass is 

 inversely proportional to the coarseness of its crystallization. To 

 appreciate this point it is only necessary to compare the form of 

 a typical mass of granite, or any granitic rock, with that of a typical 

 mass of some fine-grained exotic, like ordinary basalt. The latter 

 is straight and wall-like ; while the former is thoroughly irregular, 

 and penetrates the surrounding formations in many jagged spurs 

 and branches. 



In other words, and speaking generally, the coarsely crystalline 

 eruptives, which have cooled at great depths, occupy extremely 

 irregular cavities in the crust ; while the seats of the finely crj'stal- 

 line eruptives, which have cooled near the surface, are well-defined, 

 wall-like fissures, with parallel sides. 



It goes almost without saying that the forms of the cavities and 

 fissures filled by intrusives are determined very largely by the joint- 

 structure of the enclosing rocks ; and where these are well jointed, 

 the avenues for the liquid rock are formed mainly by the simple 

 widening of the joint-cracks. In proof of this it will suffice to cite 

 the familiar fact that dykes, if formed of fine-grained rocks, generally 

 conform closely in direction with the principal joint-planes of the 

 intersected formation. 



Now one of the most important and persistent characteristics of 

 joint-structure, especially in stratified rocks, is its regularity and 

 the really wonderful constancy in trend and dip of the joint-planes 

 over extensive districts. Therefore, fissures produced in a well- 

 jointed formation must, on the average, be far more regular than 

 those arising in a portion of the crust where the joint-structure is 

 wanting or is only imperfectly developed. The difference is precisely 

 that between the fractures of highly cleavable minerals, like calcite 

 or galenite, and of uncleavable minerals, like quartz. 



