1899] A ROCK OUT OF PLACE 315 



A Rock out of Place. 



In the July- August number of the Journal of Geology (vii. pp. 483- 

 488) Stuart Weller describes the peculiar occurrence of a small patch 

 of Upper Devonian rock in the heart of a quarry of Niagara limestone 

 at Elmhurst, Illinois. At this locality the limestone is much fractured, 

 and one of the joints is enlarged to form a cavity, triangular in section, 

 6 inches wide at the base and 16 inches high, but thinning out as it 

 passes into the rock. This cavity is filled with angular fragments of 

 the adjacent limestone embedded in a dark brown sandy matrix, which 

 contains fish-teeth, Lingula, and other brachiopods, the total suggestive 

 of a late Devonian age. From this material two new species of 

 Diplodus are described by C. R Eastman in the same number of the 

 Journal. Further, says Dr. Weller, " At the base of the triangular 

 opening, between the two beds of limestone that come in contact at 

 that point, the Devonian material extends both to the right and left 

 for several feet, forming a bed an inch or two in thickness between the 

 two limestone beds." 



The nearest outcrop of Devonian is 80 miles from Elmhurst, so that 

 the position of this patch is doubly interesting. Dr. Weller explains 

 it thus. During the greater part of Devonian time the region must 

 have been above sea - level (an inference which seems to follow 

 legitimately from the alleged age of the deposit). " The waters which 

 collected upon this land surface in part percolated through the under- 

 lying rock strata and by solution increased the size of many joint 

 cracks. At a later period, near the close of the Devonian, when the 

 sea again occupied the region, sand was sifted down into these open 

 joints, and with it the teeth of fishes which inhabited the sea there- 

 about." The opening was, " perhaps, large enough for the entrance of 

 some of these fishes." Traces of the same sandy material are seen on 

 the joint-face above the opening. 



If this explanation be true, then, as Dr. Weller phrases it, " no 

 description of any similar occurrence has been observed in the 

 literature " ; but this scarcely justifies the conclusion of the sentence, 

 " and it may be designated by the name subterranean unconformity." If 

 the mode of occurrence were at all common, if it were anything but 

 unique, then perhaps a name might be convenient. At present there 

 seems no advantage in one. Moreover, we are not convinced that Dr. 

 Weller's account is the true one. It does not allude to the occurrence 

 of clay in the other joints, and it affords no explanation of the lime- 

 stone breccia. Can Dr. Weller prove that this is not a fault-rock, in 

 which fragments of the immediately adjacent rock are mixed up with 

 fragments or washings that have fallen down the crack from the super- 

 jacent rock ? Such an occurrence is common enough, though we know 

 no technical name for it. 



