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SCIENCE. 



[N. S. Vol. V. No. 120. 



granite and the Potsdam to the Laramie, 

 and in its maximum throw must exceed the 

 aggregate thickness of the Paleozoic and 

 Mesozoic terranes ; and its completion, at 

 least, must date from relatively late geolog- 

 ical times. 



I have succeeded in tracing the sand- 

 stone dikes from the vicinity of the Iron 

 Spring, in Manitou, northwest along the 

 great fault two miles, or a little farther 

 than the sedimentary rocks extend, and 

 southeastward from Manitou, along the 

 base of the mountains, and closely fol- 

 lowing the Ute fault, to Cheyenne Canon 

 and beyond, a distance of six miles. The 

 dikes of this series vary in width up to 500 

 or 600 feet. A large dike usually follows 

 the main line of displacement, separating 

 the sedimentary rocks and granite, with 

 one to several other dikes closely parallel 

 with it in the granite. Almost without ex- 

 ception the dikes exhibit a strong south- 

 westerly hade, making angles of 5 to 

 75 degrees with the vertical, and slicken- 

 sided shear planes at corresponding angles 

 are very common. Although the rock is 

 prevailingly a fine and even-grained gray 

 and reddish-brown sandstone, identical 

 with that described by Cross, much of it is 

 decidedly coarser, and at several points it 

 is distinctly conglomeratic. In several 

 dikes, also, the sandstone is more or less 

 distinctly stratified. 



Mr. Cross has briefly discussed the origin 

 of the sandstone dikes, without arriving at 

 a definite conclusion. He recognizes that 

 these sandstone dikes are radically distinct 

 in character and origin from those described 

 by Diller in California, and asserts that the 

 known facts do not indicate the source of 

 the sand ; that the facts do show that the 

 fissures of this dike complex were filled by 

 fine quicksand injected from a source con- 

 taining a large amount of homogeneous 

 material; that such a system of fissures, 

 large and small, with their many inter- 



sections, could not remain open to be fiUed 

 by any slow process ; that the uniformity 

 and purity of the material filling fissures 

 varying from mere films on cleavage planes 

 of orthoclase grains in the granite to dikes 

 several hundred yards in width could not 

 have resulted from infiltration; and, finally, 

 that none of the sedimentary formations of 

 the region can be regarded as probable 

 sources of the material. 



My study enables me to accept all of these 

 generalizations, except the last one. The 

 most important of the facts which the true 

 theory of the dikes must explain are : first, 

 their very evident close relationship to an 

 important zone of displacement; second, 

 the homogeneity of the materials and the 

 general absence of stratification in the 

 dikes ; third, the great maximum and aver- 

 age widths of the dikes. 



The relations of the dikes to the great 

 Ute fault are indisputable. N"ot only is the 

 fault at most points closely accompanied by 

 one or more dikes ; but nowhere have I 

 been able to find any trace of the dikes 

 more than a few hundred feet (500 to 1,000 

 feet) distant from the principle line of dis- 

 placement. That these fissures, unlike the 

 relatively narrow ones described by Diller 

 in California, have not been filled from be- 

 low becomes perfectly obvious when we re- 

 flect that the inclosing rock formation is 

 a deep-seated plutonic. The homogeneity 

 and purity of the sandstone, and especially 

 the absence of feld spathic or argillaceous 

 material, make it impossible to regard the 

 dike rock as a fault breccia or as due in 

 any way to the comminution of the wall 

 rock. Ruling out this theory and infiltra- 

 tion, we are forced to the conclusion that 

 the fissures have been filled from above. 

 But of this theory two principal forms 

 naturally suggest themselves. First, the 

 fissures antedate the deposition of the sand, 

 existing as cracks in the sea-botton which 

 were filled by the slow process of sedimenta- 



