416 A Geognoslical Sketch of [Dec. 



though occurring in a magnesian lime-stone quarry, contained no 

 magnesia, but consisted of carbonate of lime coloured with iron. 



1 find a considerable difference in the chemical composition of 

 the magnesian lime-stone brought from different quarries. I shall 

 therefore give a description and analysis of the different specimens 

 I brought with me to London. My method of analysis was this : — 

 The specimen was dissolved in nitric acid, and the loss of weight 

 considered as carbonic acid. A quantity* of sulphuric acid capable 

 of saturating the same quantity of base as the carbonic acid driven 

 off was now put into the liquid, and it was distilled to dryness in a 

 retort. Distilled water was poured upon the dry residue, and 

 allowed to remain till it had dissolved the sulphate of magnesia. 

 The whole was then thrown upon a filter. The liquid that passed 

 through was evaporated to dryness, exposed to a red heat, redis- 

 solved in water, and a little alcohol being added to the liquid, it was 

 again filtered. By these repeated filtrations all the sulphate of lime 

 was separated. It was heated to redness, and weighed. The sulphate 

 of magnesia, being evaporated to dryness, was also exposed to a red 

 heat, and weighed. I conceive this method, though perhaps not 

 rigidly precise, yet as coming very near the truth ; and it has the 

 great advantage of being very simple, which in chemical analysis is 

 a point of primary importance. 



1 . Magnesian Limestone from Building Hill, near Sunderland. 



This hill constitutes a hard stratified rock, and is not the least 

 porous. Colour white, with a considerable tint of ochre-yellow, 

 chiefly passing through it in small stripes ; fracture, splintery; lustre, 

 dull, or scarcely glimmering; fragments, indeterminate, sharp- 

 edged ; translucent on the edges ; semi-hard ; readily scratched 

 with a knife; sp.gr. 2 "J9\. This variety dissolves very slowly in 

 acids, and therefore must be reduced to a fine powder in order to 

 determine the proportion of carbonic acid which it contained. Its 

 constituents were, 



Carbonate of lime 56*80 



Carbonate of magnesia 40'S4 



Carbonate of iron - 36 



Insoluble matter 2*00 



100-00 



2. Magnesian Lime-stone fro??? Hnmblcton Hill, near Sunderla?id. 



This hill is composed of porous lime-stone, full of casts of shells, 

 and marine remains, and has no marks of stratification. 



Colour cream-yellow, here and there tinged ochre-yellow, and in 

 one or two points I observed dots of peach-blossom red ; fracture 

 uneven, in some places imperfectly conehoidal ; opaque ; hardness 

 rather greater than that of calcareous spar ; scratched by fluor spar, 

 but not readily ; fragments indeterminate, rather sharp-edged ; 



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