180 ME. H. H. AENOLD-BEMEOSE ON A QUAETZ-EOCK [May 1898^ 



fication of Flatten dol omit, the various stages of whicli were seen 

 from the carbonate up to the full}" altered rock. At another locality 

 he found a similar quartzite containing Productus liorridus, and 

 considered it to be an alteration-product of Zecbsteinkalk.^ 



In 1892 Mr. Griswold described a soft Ouachita rock, in which 

 he considered that the calcite was decomposing and being replaced 

 by quartz, and also a spotted chert which was probably an oolitic 

 limestone, the grains of which were being replaced by silica." 



Two years later, Mr. Rutley, in a paper read before this Society, 

 suggested that certain quartzites were originally limestones, now 

 replaced by silica ; and, in the discussion which followed, Prof. Hull 

 expressed the opinion that better evidence of so remarkable a change 

 would be required before that author's views could be accepted.^ 



Last year Sir Archibald Geikie described a specimen of silicified 

 limestone from County Meath, in which he said that new quartz had 

 grown in optical continuity with and around grains of grit, and that it 

 was originally a gritty oolitic limestone, wherein the calcite had been 

 replaced by quartz, and the silica had been introduced in solution."^ 

 By the kindness of Frof. Watts I saw the specimen, and examined a 

 thin slice of it under the microscope. It appeared to me very 

 similar to the Derbyshire rock, but more decomposed, and may have 

 had a similar origin. 



In the Derbyshire rocks which form the subject of this paper one 

 cannot help noticing the intimate association that exists between 

 the quartz-rock and the quartzose limestone. In some localities the 

 quartz -rock alone is found, the softer rocks having been entirely 

 removed from around it hy denudation, or the junction being' 

 covered. But in other places (except in Doveholes Quarry), 

 wherever the quartzose limestone occurs, there we find the quartz- 

 rock. A quartz-vein generall}" has the quartzose limestone adjoining 

 it on either side, and lumps of quartzose limestone are often found 

 in the (]uartz-rock, and shading off into it. There is, in fact, a 

 gradual passage from the quartz-rock, through the quartzose lime- 

 stone, to an ordinary limestone, which contains few, if any, crystals 

 of quartz. In the quartzose limestone the quartz occurs not only 

 in individual crystals, but also in aggregates of crystals, which have 

 the same structure as the quartz-rock. 



Quartz, therefore, is present in the limestone under discussion as 

 numerous separate crystals, in groups of crystals, and on a larger 

 scale as veins and bosses of quartz-rock. The difference is merely 

 one of degree, and we are, I think, justified in ascribing a common 

 origin to the quartz in the quartzose limestone and in the quartz- 

 rock. 



1 Zeitschr. deutsch. geol. Gesellsch. yoI, xHi (1890) p. 371. 



2 Ann. Eep. Geol. fcjurv. Arkansas for 1890, vol. iii (1802), ' Whetstones & 

 Novac'iilites of Arkansas.' 



3 ' On the Origin oi" certain NoyacuHtes & Quartzites,' Quart. Journ. GeoL 

 Sec. vol. 1 (1-94) ])p. 377-392. 



4 Ann. Eep. Geol. Surv. U. K. for 1896 [1897], p. 58. 



