288 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



merely different phases of the same action, and it is usual to 

 find them associated; but in this instance the faults are re- 

 stricted in distribution. There is some reason to suppose 

 that the folding was of more ancient date and extended 

 through the whole area, and that the faults were superim- 

 posed over a part. 



The folds are large and small, and of various forms. The 

 larger are great elongated domes, from which the strata dip 

 in all directions. The smaller dapple the surface of the larger 

 like a ripple riding on an ocean swell. One of the chief swells 

 extends from Thousand Lake Mountain, one hundred miles 

 in a southeast direction, its remote end lying between the 

 Colorado and San Juan Rivers near their junction. Its great- 

 est width is thirty miles, and its height, if the crest had not 

 been eroded away, would be 'ZSOO feet. Its western slope is 

 gentle and its eastern steep, so that the crest runs close to 

 the eastern base. Another of the same breadth lies at the 

 north of this, extending far into the basin of the San Rafael 

 River. Its form is different, however, for its summit is broad 

 and flat, and both of its sides are steep. The trend of its 

 longer axis, too, has a different course (southwest), diverging 

 sixty degrees from that of the other. A third, of even more 

 imposing proportions, lies to the east of these, beyond the 

 field of survey; and between them are domes of smaller size. 

 All of these swells have been so demolished by the agents of 

 erosion that there remain of them only low arches of rock 

 encircled by parallel lines of inward facing cliffs. 



In the regions of faults there are great displacements and 

 small, and the small are often, just as in the other region, 

 subsidiary features of the great. The earth's crust is there 

 divided into a great number of oblong blocks with vertical 

 sides, and these blocks have slipped out of their original 

 places, some going up, some going down, many being tipped 

 this way or that, as though an end had caught while it was 

 moving. At the north the tops of the higher blocks have 

 been worn away, and their positions and limits can be as- 

 certained only by careful study of the rocks. But at the 

 south the whole surface was covered by a thick lava sheet 

 before it was divided into blocks ; each block is protected 

 from erosion by its cap of tough, hard lava ; every mount- 

 ain is a block upthrust, every valley is a block depressed ; 



