part 2] LIMKSTONE OF THK l>KlC"K><rKRSHI HK COA I>FI Kf-l). 105 



The amount of five ealeite is always small, as shown l>y slides 

 stained with Lem1)erL!:"s solution. 'Hiese doloiniti's are mostly 

 of minutely erystalline texture, mieroseo])ie slides showiui^ more or 

 less idiomor])hie rhoml)ohedra, the (uitlines »)f whieh are seen more 

 clearly under a hi«^h power (see PI. X, WiX- '->). 



Limonite is jnvsent interstitially, hut tlu-re are no zonal inelusions 

 of luematite eharaeteristie of the red dolomites deserihed helow. 



Organic struetures have heen re))laced hy dolomite, or occur 

 as dolomitie easts; and it is ])rohal>le that many fossil structures 

 have heen ohliter.ited altoijfetlu'r. The dolomitizution of thrse heds 

 a)>i)ears to he so eom))h'te, that examination of the fossils and their 

 matrix shows no 'selective' dolomitization. 



The yellow and i^rey dolomites of Ticknall are very similar in 

 mineral«)i,Meal structure and ehemieal eonqxtsitinn to the massive 

 dolomites of the eastern iidiers, hut those at Tieknall are coarser in 

 «,M-ain and «|uite harren of fossils (see PI. X, tii^s. I & 2). 



In all of these rocks tliere is a j^eneral ahsence of tlxjse features 

 which usually indicate that the dolomitization is due to the in- 

 riuence of waters of a |M)st-Carhoniferous date, and Held evidence 

 appears to supix)rt the view thut these dolomites are of the 

 kind descrilx-'d as ' con tcm])ora neons ' hy some authors. They 

 occur in definite l>eds, do not pass laterally into unaltered lime- 

 stone, and their convei'sion into dolomite does not a]ipear to l)e due 

 to faultiuir. 



The thinly-l)e(l(lcd red dolomites of Brecdou 

 and Brcc'chni Cloud. 



Only in the up})crmost heds of Hreedon-on-the-Hill. and of 

 Hreedon Cloud is there evidence of dolomitization due to the 

 influence of the Trias. 



The rock is pink or red, and the dolomitization isvariahle in any 

 particular stratum. Tlie amount of ha'uuitite sometimes exceeds 

 5 per cent. Mici-oscojiic .sections show relatively larice idio- 

 morphic crystals, with zonal inchisions of ha-matite, and having' an 

 almost clear outer zone (see PL X, fii^. 4). 



The only or^nmic structures of common <tccurrence are erinoid 

 stems, which remain mostly in an undolomitized condition. Here we 

 note 'selective' dolomitiz.iition, in which the matrix is d<domitized 

 while echinoderm structures remain unaltered. In no ease have 

 I found selective dolomitization, in wliich the rhomholiedra have 

 ileveloped in ori^anic structures and not in the matrix, as is the case 

 in s<nne vein-d(»lomitvs of otljer localities. Ihit the eharai-teristic 

 zonal hsematite inclusions which are always j»resent, anil the |>;itchy 

 nature of the (h)lomitiz-ition, leave little douht that these n*d 

 «l(domit4's may he considered to \>r of ' suhse«juent ' oriirin. 



