234 ME. C. T. TRECHMAKNT ON THE LITHOLOGY [June 1914, 



In the analyses tabulated in this paper, care was taken, both to 

 ascertain the degree of alteration which the rocks had undergone, 

 and to obtain average samples of the material free from secondary 

 calcite or other crystals. 



The following criteria were used in the recognition of beds which 

 have escaped segregation or granular collapse : — 



(a) General well- stratified character of the rock, whether massive or fissile, 



compact or friable. 



(b) Fineness and uniformity of grain, together with the absence of secondary 



calcite or of powdery dolomite. 



(c) The presence of casts of fossils in uncrushed or undisturbed condition. 



(d) The presence in undisturbed condition of hollow cavities, presumably 



left after the solution of sulphates. 



In a previous paper, 1 I endeavoured to point out the effect of the 

 conversion of anhydrite formerly present in the formation into 

 gypsum and its subsequent removal by solution, on the breccia tion 

 and degradation of parts of the Magnesian Limestone ; as also its 

 influence on the processes of segregation and liberation of granular 

 dolomite, and on certain features connected with the tectonics of 

 the formation. 



The present paper is a sequel and extension of that just men- 

 tioned, based on a much closer enquiry into the composition of 

 various beds. This being the case, a certain repetition of points 

 already touched upon in the previous communication may perhaps 

 be excused. 



Repeated analyses 3 show that the Magnesian Limestone is 

 essentially a dolomitic rock, the chief constituents of which may 

 be roughly indicated by the following synopsis, though with no 

 reference to their varying proportions : — 



Previous Composition. Present Composition. 



Dolomite. Dolomite. 



Calcite. Calcite. 



Anhydrite and gypsum. 



Ferrous carbonate. Iron sesquioxide. 



Manganese carbonate. Manganese dioxide (dendritic). 



Sandy residue and colloidal silica. Siliceous residues. 



Sulphides. Pyrites (free or enclosed in quartz). 



Micaceous and heavy minerals. Micaceous and heavy minerals. 



Organic matter. Bituminous matter. 



1 Q. J. G. S. vol. lxix (1913) pp. 195-98. 



2 The process of analysis employed was the direct gravimetric one, with a 

 double precipitation of the calcium and magnesium as oxalate and phosphate 

 in well-diluted solutions. Confirmatory analyses were made in several cases. 

 The results are further reckoned to terms of calcite and dolomite. Several of 

 the rocks were sliced, and stained with Lemberg's solution. The quantity of 

 superfluous calcite in the more dolomitic rocks in most cases agreed with that 

 indicated by the degree of coloration on staining. 



Samples were powdered and dried at 120° C. before analysis. Some esti- 

 mations of carbon dioxide were also made, but were found to agree so closely 

 with the calculated amount that this was neglected later. 



