382 MR. F. RUTI/EY ON THE ORIGIN OP [-A-Ug. 1 894, 



as the acid dissolved and disintegrated it. The deposit, when 

 examined under the microscope, was found to consist of minute 

 rhonibohedra, some of which were considerably, and others slightly 

 eroded, while many exhibited perfectly sharp angles and edges 

 (PI. XIX. fig. 6). 



It seems quite possible that such a limestoue might, through 

 natural causes, become almost wholly dissolved and replaced by 

 silica, the replacement going on gradually and keeping pace with 

 the dissolution of the limestone, a rhombohedron becoming here 

 and tbere enclosed in the siliceous matter and thus protected from 

 further erosion. That such was the case in the Ouachita stone 

 is rendered extremely probable, from the fact that many of the 

 cavities have irregular boundaries, such as would have resulted from 

 the enclosure of partially eroded rhombohedra or small groups of 

 rhombohedra. In PI. XIX., figs. 9 & 10, two such cavities are 

 shown. Sometimes cavities occur which have evidently been occupied 

 by a group of ten or twelve coherent crystals. I think it must be 

 admitted that the forms of these cavities give evidence of erosion, 

 rather than of arrested crystallization of the mineral which once 

 filled them. 



In the next place, let us assume that the original limestone was 

 not a true dolomite, but merely a dolomitic limestone. Then, since 

 calcite is more readily soluble than dolomite, it is easy to imagine 

 that the whole of the calcite might become dissolved and replaced 

 by silica, the latter enveloping the less readily soluble rhombohedra 

 of dolomite. If a small amount of limestone can be replaced by 

 silica, as in the layers and nodular bands of chert, met with in the 

 Carboniferous Limestone, there seems no reason why, given a sufficient 

 supply of silica in solution, thick beds of limestone should not be 

 wholly replaced. 



That it is unsafe to deny, in all cases, a detrital origin to siliceous 

 beds associated with limestones is rendered sufficiently evident from 

 the occurrence of thin beds of metamorphosed sandstone or quartzite 

 interstratified with limestone at Modoc Peak in the Eureka district. 

 This saudstone is described by Prof. Iddings as somewhat micaceous 

 and graduating from an extremely fine-grained to a coarse-grained 

 rock " having the mineral composition and structure of a micro- 

 granite." x Judging from the figure given in his memoir, which re- 

 presents a section magnified 33 diameters, the rock is a quartzite, 

 or, as it is described, a quartz-conglomerate, of somewhat coarse 

 texture and in no way resembling siliceous rocks of cryptocrystalline 

 character, such as Arkansas stone, Ouachita stone, flint, or chert, 

 none of which exhibit characters which a sandstone even of the 

 most altered t3 T pe would possess. The Modoc Peak quartzite, on 

 the other hand, consists of rounded grains of quartz cemented by a 

 secondary deposit of silica, and is unquestionably a sedimentary rock. 



1 * Geology of the Eureka District, Nevada,' by Arnold Hague, Monographs 

 "U.S. Geol. Surv. vol. xx. (1892), Appendix B. by J. P. Iddings, p. 346, and 

 fig. 3, pi. iv. 



