ON THE FORMATION OF CAVITIES IN ROCKS. 15 



roll, and contained water about one hundred feet below the top. 

 Its reproduction of the conditions of a vein, with horse, pinches 

 and swells, devious course, and all, is striking. The photograph 

 was made by Mr. G. K. Gilbert, of the United States Geological 

 Survey, and to his courtesy its use is due. 



FIG. 2. Open fissure in the Aubrey limestone (Upper Carboniferous), 25 



miles north of Canon Diablo station, Ariz., on the A. & P. R. R. 



Photographed by G. K. Gilbert, 192. 



While it is true that in many regions the folds and fractures 

 have resulted in this simple way, and exhibit the unmistakable 

 course through which they have passed, yet geological structure 

 is by no means always so clear. Extended disturbances, great 

 faults and displacements, combined with folds and the intrusion of 



