i64 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 



The effects predicted have now become prominently objection- 

 able in several instances* 



AVe have some excellent gray granites within ready access, 

 and many of these are tough, tine grained, fairly homogeneous, 

 with texture and mineral components well adjusted for tooling 

 and polishing satisfactorily. But there are areas in which the 

 lines of jointing and faulting and other structural features 

 have so shattered the masses as to make impossible the procur- 

 ing of suitable blocks for architectural design. Occasionally 

 these deposits may be utilized for rip-rap, rubble masonry and 

 other structures not demanding regular courses. Still, it is not 

 always safe to employ these without careful examination of the 

 local conditions by an expert. Lines of weakness not apparent 

 from cursory observation may, perhaps, be detected in the lay 

 of the deposit or the geologic conditions of the neighborhood. 

 Sometimes mineral streaks giving no evidence of cleavage will 

 afterwards develop fracture lines in actual use. Again, certain 

 minerals, easily oxidized or disintegrated by wear, may, at first, 

 offer an attractive or apparently serviceable surface, which, 

 cannot long withstand the influence of city environment. 



These features must be judged on the ground by experts, 

 familiar not only with the requirements of construction, but 

 also with the geologic and engineering conditions concerned 

 in each particular problem. It is a prevalent opinion that ex- 

 posures with the rock badly shattered at surface are liable to 

 show improvement in this respect at greater depth. This idea 

 is commonly erroneous, except as it may apply to deposits 

 wheer atmospheric agencies have been responsible for the sur- 

 face conditions. And, usually, then the bringing of unexposed 

 portions to air will produce similar results. Per contra, it is. 

 not always certain that surface exposures of wholly satisfac- 

 tory material will hold out the same in depth, although the 

 chances are, perhaps, more in favor of such occurrence than 

 otherwise. 



There are some black granites in the foothills of the San 

 Gabriel Mountains, which ought to find limited use in orna- 

 mental work. For parts, or the whole of monuments, and for 

 some decorative purposes, they are valuable on account of their 

 toughness, variegated appearance and susceptibility of polish. 

 They are usually coarse-grained and do not tool as readily, in 

 consequence. There is some variety in these particulars. Some 

 of our local monument dealers are using fine-grained black 



*I refer to the staining, really the rusting, of the black minerals along- 

 the seams, due to oxidation of the iron-bearing minerals. 



