CIRCULATION OF AQUEOUS SOLUTIONS. 1025 



The strongest evidence of the vigorous and extensive character of the 

 circulation in the belt of cementation is the fact, so strongly emphasized 

 on pages 562-565, that cementation is universal in it. Wherever porous 

 rocks have long- remained in the belt of cementation an enormous amount 

 of material has been deposited by underground aqueous solutions. In 

 most cases where the porous rocks have been deeply buried in this 

 belt for geological ages cementation is nearly or quite complete. Thus 

 great sandstone formations have been transformed to quartzite by deposi- 

 tion of interstitial silica. In the San Juan district of Colorado great tuff 

 formations of Tertiary age have been completely cemented. In order to 

 get an idea of the original porosity of this formation it should be compared 

 with the very recent porous cinder cones made up of bombs and lapilli, 

 such as occur at many places in the West, as for instance near Flagstaff, 

 Ariz. These cinder cones are so porous that rain water, however great its 

 amount may be, passes into them as it would into a sieve, and therefore 

 performs no work of erosion. Indeed, it seems as if the process of erosion 

 of such deposits could scarcely begin until the openings had been partly 

 filled by the process of cementation. Yet in the San Juan district of Colo- 

 rado thousands of meters of tuffs originally as porous as these have been 

 so perfectly cemented that the microscope scarcely discovers an opening 

 except those which have been produced by recent fracture since cementa- 

 tion. The fillings of the major and minor openings are continuous physic- 

 ally and are composed of the same kind of material. No one who has 

 studied such a formation as this can doubt that the general cementation 

 and the filling of the fractures were performed by the same agent. 



In general, in the rocks which have been long within the belt of cemen- 

 tation, the innumerable bedding partings, faults, joints, fissility and breccia- 

 tion openings have been closed. The complete cementation of the more 

 porous sandstone and tuffs requires the deposition of mineral material to 

 the extent of 30 to 40 per cent of the volume of the rock. It has been 

 shown (pp. 571-572) that to have accomplished this work the amount 

 of water which circulated through the rocks probably in most cases must 

 have been 100,000 or more times as great as the material deposited. 

 Further, in case of the San* Juan tuffs, the enormous amount of water 

 required to deposit the immense mass of cementing material passed through 

 the rocks since early Tertiary time. 



MON XLVII — 0± 65 



