44 Harker : Petrology in Yorkshire. 



Geological Society of London in 1879, Dr. Sorby showed that 

 the two forms are to a great extent characteristic of the hard 

 parts of different sub-kingdoms, classes, or genera, although 

 certain organisms secrete both ; and further that in many 

 limestones shells originally of aragonite have been converted 

 to the more stable form, calcite. 



The question is evidently one which has a bearing on biology, 

 and perhaps on climatology. It has recently been reopened, 

 with the result of showing that there is still much to be learnt 

 concerning both the mineral constitution of shells and the 

 conditions of preservation of fossils. I mention the matter 

 chiefly to point out how much research is facilitated by im- 

 proved methods of working. Sorby discriminated calcite and 

 aragonite by means of their different specific gravities. The 

 use of a heavy liquid would now make this much easier ; but 

 a simpler test is afforded by boiling in Meigen's solution,* 

 which stains aragonite violet, but leaves calcite unchanged. 



It would be easy to cite other lines of inquiry in connection 

 with sedimentary strata, which were opened out by Dr. Sorby, 

 but still await final settlement. There is, for instance, the 

 conversion of limestone to dolomite. We know that a cal- 

 careous deposit may be dolomitized almost contemporaneously 

 with its accumulation or, on the other hand, by an accession 

 of magnesia in some form at a long posterior time ; but the 

 relative importance of the latter factor, the conditions under 

 which the process is effected, its possible relation to depth and 

 pressure, are questions still agitated. Might not some light 

 be thrown upon them by carefully conducted study of the 

 Permian limestone of the South of Yorkshire or the Carboni- 

 ferous of the West ? Here again we may note that the practical 

 differentiation of' calcite and dolomite is now made easy by 

 a coloration test : Lemberg's solutionf stains calcite, but not 

 dolomite. Among cognate problems is that of the replacement 

 of limestone and calcareous fossils by silica, a change which 

 may be observed in various stages in some of our Corallian 

 strata ; and again the conversion of limestone to ironstone, 

 which has made possible the principal industry of the Cleve- 

 land district. 



{To be continued). 



* A dilute solution of cobalt nitrate, free from iron. 

 t A solution of aluminium chloride, heated with logwood. The 

 solution is applied to the substance for 5 to 10 minutes. 



Naturalist, 



