228 W. CROSS — INTRUSIVE SANDSTONE DIKES IN GRANITE. 



little clear feldspar, either orthoclase, microcline or plagioclase. Many of 

 the quartz grains contain inclusions of apatite and zircon and a few small, 

 round inclusions probably of hornblende, but neither mica, amphibole 

 nor pyroxene has been seen in sand particles. The average size of the 

 quartz grains is less than 1 mm., and in many cases is less than 0.5 mm. 

 A few larger grains are seen in some dikes. 



The induration is mainly due to limonite in small flakes, and in less 

 degree to muscovite in minute leaflets wrapped about the grains. Occa- 

 sionally there appears to be some secondary silica, but a distinct enlarge- 

 ment of the quartz grains is rare. Tourmaline and other corresponding 

 secondar}^ minerals are wholly absent. Minute fluid and gas inclusions 

 are common in the quartz. 



The mineral composition of the sand and the size and form of the 

 grains are practically the same in all dikes, large and small. Occasion- 

 ally angular fragments of the adjoining granite are mingled with the 

 sand, but cases are extremely rare where any difficulty arises in distin- 

 guishing this material from the sand of foreign origin. 



Origin of the Dikes. — As far as the writer is aware, no other occurrence 

 of sandstone dikes in granite has ever been described. As dikes of sand- 

 stone they may be compared with the remarkable occurrences in Cali- 

 fornia described by Diller * in the first volume of this Bulletin ; but the 

 dikes of this latter occurrence were in shales of a great sedimentary com- 

 plex of Cretaceous age, and they were parallel to a S3^stem of jointing 

 planes in the strata. Moreover, Diller noted that below the horizons 

 occupied b}^ the dii^es there occurred sandstone strata of a composition 

 identical with that of the- dike-rocks. The very plausible theory pre- 

 sented by Diller was that the fissures represented by the dikes were 

 formed by earthquake shock, and that the sand was injected as quick- 

 sand into the fissures under hydrostatic pressure from unconsolidated 

 water-bearing sand layers below. He observed many minor details sup- 

 porting this theory. 



It is clear that the sandstone dikes described above are far more diffi- 

 cult to explain than those of California, in that the known facts do not 

 indicate the source of the sand ; yet the physical and mechanical facts 

 do seem to show that the fissures of this dike complex were filled by fine 

 quicksand injected from a source containing a large amount of homoge- 

 neous material. On the one hand, it is impossible to suppose that such a 

 system of fissures, large and small, with their many intersections, could 

 remain open to be filled by any slow process, and, on the other, it is 

 equally imi:)ossible to believe that the uniformity and purity of the mate- 



■ * J. S. Diller, Sandstone Dikes: Bull. Geol. Soe. of Am., vol. 1, 1889, p. 411. 



