Vol. 50.] ceetahs" novaculites and qttaktzites. 381 



and inclusions in certain siliceous rocks to which the name 'quartzite' 

 is applied by some authors, and ' chert ' by others. 



Some of the cherts derived from the Carboniferous Limestone of 

 Belgium, and described by Prof. Renard, contain rhombohedral 

 crystals which, although very minute, give distinct reactions for 

 magnesia, and these Prof. Renard regards, apparently with good 

 reason, as crystals of dolomite. 



In a section of a pebble from the conglomerate of Purtiall, in the 

 Leccan, I have noticed the occurrence of similar rhombohedral 

 crystals. Portion of a section of this pebble is represented in 

 PI. XIX. fig. 4, as seen between crossed nicols. The polarization- 

 picture afforded by the quartz in this part of the section is suggestive, 

 as I have already stated, of a cryptocrystalline structure, but the 

 marginal portions of the section give distinct proof that the rock 

 consists of small crystals of quartz and, could the preparation be 

 rendered as thin in all parts as it is on the margin, it would doubt- 

 less appear, in polarized light, as a uniform mosaic of distinctly 

 individualized quartz-crystals. One of those on the margin, repre- 

 sented in PI. XIX. fig. 7, gave a positive uniaxial interference- 

 figure. 



The occurrence of small rhombohedral crystals of dolomite in " a 

 flinty, grey or dark-grey jaspilyte from the shaft at the Breitung 

 mine," Minnesota, has been pointed out and figured by Prof. N. H. 

 Winchell. 1 



It has been considered doubtful whether the cavities in the 

 Arkansas novaculites were originally occupied by calcite or dolomite. 

 If these rocks represent the replacement of limestone by silica, it 

 may, I think, be assumed with good reason that the limestone so 

 replaced was either a dolomite or a dolomitic limestone. 



There seems nothing unreasonable in such a supposition, since 

 dolomites of Archaean age are known, and Huronian dolomites and 

 dolomitic limestones occur in Michigan. 2 



Let us, in the first case, suppose the replaced rock to have been a 

 dolomite. It is well known that dolomites consist, as a rule, of 

 minute rhombohedra. A section cut from a specimen collected at 

 Matlock, from one of the magnesian limestone-beds which there 

 constitute part of the Carboniferous Limestone system, consists 

 almost entirely of such small rhombohedra (PI. XIX. fig. 5). An 

 analysis which I made of tbis specimen showed it to be almost 

 identical in composition with the typical dolomites of the Magnesian 

 Limestone series, the calcium carbonate amounting to 51-25 and the 

 magnesium carbonate to 4218 per cent., the remainder consisting 

 mainly of silica, with a little iron, alumina, and water. A small 

 fragment of this specimen, whon placed in dilute hydrochloric acid, 

 let fall a fine granular deposit, the grains falling from the fragment 



1 ' The Iron Ores of Minnesota,' Geol. & Nat. Hist. Surv. Minn. Bulletin 

 no. 6 (1891) p. 77, and pi. viii. 



2 Oredner, ' Elemente d. Greologie,' 3rd ed. 1876, p. 373. 



Q. J. G. S. No. 199. 2 d 



