FISSURES. 51 



than the other, but sometimes the rock is divided into square columns, ^vhich 

 look as if they had been cut and smoothed by stone masons. The prevalence 

 and the strongly marked character of these vertical fissure systems led me 

 to devote to them most of my attention. They are so common that one sel- 

 dom travels more than a mile or two over granite without encountering them, 

 and I estimate that they are distinctly developed in more than one-half of 

 the area occupied by granular rocks. 



Fissures of more than one kind appear in some cases to penetrate from 

 the granite into overlying andesite, but the lava flows are for the most part 

 very heterogeneous agglomerates, and w^hen this is not the case they are ajot 

 to be columnar. They are, in short, in all respects ill adapted to the study 

 of orogeuic movements. 



The Fissures are Fault Planes. — Though the fissures in the granite never 

 gape and are usually only cracks, they are not mere partings or joints but 

 true fault planes. In innumerable cases they show excellent slickensides 

 and very often the amount of dislocation can be demonstrated. The fault- 

 ing has beei> attended by intense compression, so that at some points the rock 

 for an inch or two from the principal plane of motion has been sheared to a 

 mass resembling slate. This is comparatively rare, but it is often manifest 

 that, for some distance from a pronounced fisssure, lines of weakness have 

 been developed in the rock at intervals of a quarter of an inch, or there- 

 abouts. These lines grow less distinct as the distance from the fissure in- 

 creases, and disappear a foot or two from the fissure. 



Frequency of the Fissures. — In areas intersected by the vertical fissure sys- 

 tems the horizontal interval between the parallel planes is variable. Occa- 

 sionally they are a couple of hundred feet apart, but this is rare. There can 

 hardly be said to be an inferior limit to the horizontal intervals, for there 

 is every gradation from almost microscopic frequency upwards. It seems 

 possible, however, to distinguish two sets of fissures among members of a 

 single parallel system. A portion appear to represent original fractures of 

 the granite, while the^mere act of faulting under strong compressive stress 

 has produced a second set subordinate to the first, and of course parallel to 

 them. An exact determination of the frequency of the primary fissures is 

 impossible, but careful estimates on the ground led me to believe that the 

 horizontal interval between them averages about five feet. 



Age of the Fissures. — There are certain dikes of a highly feldspathic, nearly 

 white, granitic rock which intersect the darker granites of the Sierra and the 

 areas of metamorphic strata sometimes imbedded in the granite. Though 

 the age of these dikes requires more investigation, I have reasons for believ- 

 ing it to be early Cretaceous. These dikes are more ancient than the fissure 

 systems under discussion. There is much to show that California was in a 

 very quiet condition from the Gault, or thereabouts, to the close of the 



