Vol. 54.] IN THE CARBONIFEROUS LIMESTONE OF DERBYSHIRE. 179 



Bakewell, described in the Geologieal Survey Memoir of North 

 Derbyshire, 2nd ed. (1887) p. 166. 



Under the microscope the chert-nodnles consist of a cryptocrystal- 

 line quartz-mosaic, with flecks of calcite distributed through it. 

 The ' siliceous limestone ' consists of a microcrystalline quartz- 

 mosaic, often passing into cryptocrystalline structure which contains 

 small patches of the former. The threads of silica consist of a 

 mosaic of clear quartz-grains, without -dnj crystalline outline, and 

 not elongated. They traverse a cryptocrystalline quartz. 



This siliceous rock is probably a replacement of limestone by 

 silica, which has not crystallized out like that in the quartz-rock 

 described in the former part of this paper. It is more allied to 

 chert. 



IV. The Quartz-rock is a Limestone which has been 



REPLACED BY CRYSTALLINE QuARFZ. 



. It is well known that quartz- crystals occur in limestone : some re- 

 ferences will be found in Mr. Wethered's paper.^ It does not appear, 

 however, that the crystals have been previously found in large 

 quantities. Out of 30 lbs. of limestone from Caldon Low dissolved 

 by Mr. Woodcroft, 3 ounces only of quartz-crystals were obtained." 

 In the thin sections of limestone which I have examined from other 

 parts of Derbyshire the quartz-crystals are seldom present, while in 

 the limestone in contact with the quartz-rock they often form half, 

 or a greater portion of the thin slice. 



In 1881 Lang described a quartzite which consisted of inter- 

 locking grains of quartz. In one locality it contained fossils.^ 



In the following year Groddeck described a silicified Devonian 

 coralline limestone, which under the microscope consisted of a very 

 fine-grained mass of quartz.^ 



In 1887, in the Geological Survey Memoir of North Derbyshire, 

 the Pindale blocks were mentioned, and the following explanation 

 was given of their origin : — ' At the head of Pindale they distinctly 

 occur as a bed in the limestone, and the vast number of loose blocks 

 seem to have been left by the removal of the limestone round them. 

 They seem to have been a very cherty limestone, or in some cases a 

 bed of chert with patches of limestone. As a result of aqueous 

 metamorphism the limestone was removed, and its place taken by a 

 white sand, resulting from the decomposition of chert' (p. 130). 



In 1890 Loretz drew attention to some loose quartz-blocks in the 

 district of Schwarzburg, which he described as due to the silici- 



^ * On Insoluble Residues obtained from the Carboniferous Limestone Series 

 at Clifton,' Quart. Journ. Geol. Soc. vol. xliv (1888) pp. 19 1, 193. 



2 W. J. Solla<, ' On the Plint-Noclules of the Triiuiagham Chalk,' Ann. & 

 Mag. Nat. Hist. ser. 5, vol. vi (1880) pp. 445-447. 



3 * Ueber Sedimentar-Gesteine aus der LJmgegend von Gottingen,' Zeitschr. 

 deutsch. geol. Gresellsck. vol. xxxiii (1881) p. 217. 



* ' Zur Kenntniss des Oberharzer Culm,' Jahrb. d. k. preuss. geol. Landesanst. 

 1882, pp. 59-62. 



