G. K. Gilbert — Oriijin <>f Jointed Structure. 53 



found to contain a greater number of joint systems than the 

 newer, and that, cceleris paribus, there would be found a more 

 or less gradual increase in the complexity of joint division 

 from new rocks to old. Such, however, is not the case. Ee- 

 stricting our attention to those rocks which lie nearly level, 

 we find that Paleozoic bit more than three or 



Quaternary clays of the Salt Lake Basin ha 

 In the range of my own experience the rocks freest from joint- 

 are the massive, cross-iaminated, Triassic and Jurassic sand- 

 stones of the Colorado Plateaus, and these strata afford the 

 only localities in which I have ever observed a single joint 

 system. Indeed, the record of single systems of joints is so 

 rare that I am disposed to question my own observation and 

 suspect that the minimum number of coincident joint systems 

 is two. However this may be, it is certainly an objection to 

 'on theory that, while the number of joint-systems 

 formations of all ages is frequently no more 

 than two, it is rarely or never one. 



Now, unless we include the suggestion that joints have a 

 magnetic cause, or the absolutel . jis that they 



are due to shearing force, there seem to be only the two 

 hypotheses above discussed to account for them. The most 

 competent writers who have treated of them have classed them 

 either with shrinkage cracks or with slaty cleavage, ascribing 

 them on one theory to ing and on the other to 



mechanical pushing. If the considerations here adduced have 

 weight, then neithi I fcory, and the problem 



is an open one. It is certainly hard to correlate the parallelo- 

 pipedons into which the clays of the Salt Lake Desert are 

 divided with the polygonal prisms normally arising from 

 : and it is equally hard to admit that the clays have 

 been subjected since their deposition to coercive pressure from 

 two independent directions. 



In my judgment it is proper to conclude, first, that joints are 

 not due to shrinkage, and second, that the theory which regards 

 them as identical with slaty cleavage and ascribes both to com- 

 pression is untenable. If pressure and compression suffice for 

 the explanation u f slaty cleavage, then jointed structure is 

 something distinct from cleavage and needs an independent 

 i. If joints and cleavage are merely diverse exam- 

 ples of the same general structure, then the theory of slaty 

 ■Vavaire which has been so widely received fails to compre- 

 hend all the facts and needs to be revised. 



