•Jill THE PERMIAN FORMATION. 



whole or fragmentary remains of these organisms ; but this in no way 

 disproves the direct chemical origin — and still less proves the organic 

 origin— of the major portion of these rocks. The finer grained Magne- 

 sian Limestone dolomites, such as those of Mansfield Woodhonse, 

 Cresswell, and Kirkby, (and I believe the same remarks will apply to 

 the more coarsely crystalline also,) when a microscopic section has 

 been made, are seen to consist of uniformly-sized and shaped rhombic 

 crystals of dolomite, more or less stained with iron oxide, and inter- 

 I with a few scattered quartz grains. (See Fig. 4a.) Had dolo- 

 mitisation of these rocks taken place after their formation, this could 

 only have been caused by extensive and intimate infiltration of waters 

 holding mineral matter in solution, in which case we should surely 

 have seen some indications of this process. But we do not. We see no 

 i if infiltration, no (microscopic or larger) veins of calcite or other 

 mineral. but regular crystals of dolomite, which by polarised light, 

 appear to be simple crystals of homogeneous structure. Then again, 

 the Upper Limestone, (or Brotherton Beds of Yorkshire,) which is 

 only separated by a few feet of marl from the Lower Magnesian Lime- 

 stone, and sometimes rests directly upon that rock, is, as a general 

 rule at any rate, non-dolomitic. The underlying Marl Slates, too, are 

 by no means generally dolomitic throughout. By what agency could 

 the Lower Magnesian Limestone be afterwards dolomitised, and the 

 Marl Slates and Upper Limestone escape the process? 



In the face of the foregoing facts, it appears to me impossible to 

 suppose tbat the Magnesian Limestones of the north-east of England 

 were mainly organic in origin, and have long subsequently undergone 

 the wholesale dolomitisation and crystallisation which that theory 

 |. upposes. I believe, on the other hand, that these crystalline 

 dolomites were originally formed as such, and that the proportionate 

 value of organic life in building them up is approximately given by the 

 small aim. nnt that is now visible to the naked eye or revealed b\ the 

 microscope. Long after their deposition, the coarsely crystalline 

 dolomitic beJs appear to have been subjected to the infiltration of 

 carbonated waters, whereby the excess of carbonate of lime and the 

 calcareous fossils were removed, leaving the mass of the rock as we 

 now so commonly find it. porous in texture, and with only the hollow 

 i) (or moulds) of the fossils left. 



I ii i as now proceed to consider the probable physical sequence of 

 events that took place in the north-east of England in Permian 

 times. (>>) 



! i Permian area of the north-east of England appears to have 

 original] ted of a single pretty level flat-bottomed basin, or of 



md suggestive phenomenon mel with in Dolomitic Lime- 



■ 



bhaf from the necessarilj imperfect Btate of 

 out Information on the subject, the following remarks are largely of a speculative 



nature. 



