REPORT OF COMMITTEE ON PHOTOGRAPHS. 629 



The folds are the results of horizontal pressure applied to the ends of flat strata 

 which were confined on four sides and at the bottom, and could rise only by lifting a 

 load of bird shot. 



The materials composing the models were mixtures of plaster of Paris and beeswax 

 and Venice turpentine in varied proportions, which resulted in strata of varied plas- 

 ticity. 



Each pile of strata was compressed from two to five times and photographed at each 

 stage of compression. The photographs therefore represent successive stages in the 

 development of folds and faults. 



254. The vertical datum lines, continuous in the left end of the model, were broken 



across, as the strata near the right-hand end slipped past each other in folding. 

 Size, 6x8 inches. 



255. Further compression of No. 254, resulting in a fault which follows the plane of 



inflexion between the anticline and syncline, but which was not preceded by 

 thinning or stretching of the strata. Size, 6x8 inches. 



256. A series of thinly laminated beds above conform closely to the flexure of a massive 



bed below. The former was incompetent to lift the load of shot ; the latter is 

 competent to do so. Size, 6x8 inches. 



257. Second stage of experiment with No. 256. Size, 6x8 inches. 



258. Third stage of experiment begun with No. 256. Size, 8 x 10 inches. 



259. Another view of third stage of experiment with No. 256, showing mammillated 



surface induced by pressure of shot. Size, 8 x 10 inches. 



260. 261, 262, 263, 264. Five stages of development of folds in a pile composed of a 



massive layer (white) overlaying a series of thin layers. Size, 6x8 inches. 



265, 266, 267, 268, 269. Five stages of development of folds in a pile composed of 

 strata diff'ering less markedly than in series 260 to 264 in thickness and con- 

 sequent diff"erences of rigidity. The load was uniformly distributed and the 

 three folds similar in dimensions. Size, 5x7 inches. 



270, 271, 272, 273, 274. Five stages of development of folds and faults in a pile of 

 strata composed of a thick bed above and many thin ones below. Differential 

 folding within the principal anticline led to faulting within the fold, and the 

 displacement was deflected upward into the axial planes. Size, 5x7 inches. 



275, 276, 277, 278, 279. Five stages in the development of folds in a pile composed of 

 a hard bed above and more plastic, thinner layers below. The latter thinned 

 where the upper stratum and load rested heavily and thickened where the upper 

 stratum carried the load. Size, 5x7 inches. 



280. Complex folds and indirect faults produced in an asymmetrical mass. Strata on 



the left massive, strata on the right thinly laminated. Size, 6x8 inches. 



281 , 282, 283, 284. Anticlines developed at base and top of an assumed monocline, and 



modified by cutting off the tops of the arches between each two stages of com- 

 pression. The last stage of this model was exhibited at the Washington meet- 

 ing, in December, 1890. Size, 5x7 inches. 



Unclassified. 



285. Conception bay, Newfoundland. Size, 6x8 inches. Photographed by C. D. 



Walcott, 1888. 



286. Hydraulic mining, Cherokee Flat, Butte county, California. Size, 8 x 10 inches. 



Photographed by J. S. Diller, 1885. 



287. Characteristic timber-line growth of stunted spruces, at an elevation of 12,100 



feet, head of Cement creek, Gunnison county, Colorado. Size, 8 x 10 inches. 

 Photographed by C. Whitman Cross, September 20, 1885. 



