internal Structure of the Magnesian Limestone. 85 



of the same greyish white colour, but are of a softer and more earthy texture, and on that account 

 are not used for building. 



7. In following the formation from the last-mentioned locality towards the north, there are for 

 nearly thirty miles very few traces of the fine white dolomites. But they reappear at Huddlestone 

 near Sherburn, and on the north-east side of Bramham Moor. At the former place is a quarry 

 about forty feet high, exposing ten or twelve great irregular beds or tabular masses. Some of 

 them have the lustre and texture of the Roach Abbey stone, but in general they are less crystal. 

 line ; and in one part of the quarry they pass into a compact yellowish white rock which exhibits 

 the conchoidal fracture and external characters of a perfect Alpine limestone*. In the quarries 

 of Bramham Moor the best beds are yellowish white, very fine-grained, of dull glimmering lustre 

 and less crystalline than the Iluildlestone beds. Many of them are veined and cellular and pass 

 into common earthy varieties of magnesian limestone. The two last-mentioned quarries have 

 supplied materials for the construction of some of the finest Gothic buildings in our island. 



To the north of the river Wharfe, the whole formation of magnesian limestone does not I 

 believe, afford a single quarry of dolomite which deserves notice from its value as a buildino- stone. 



In the description of the two preceding modifications, the term '^dolomite" has been used in a 

 somewhat extended sense ; for the specimens are certainly not all composed of a definite triple 

 salt, in which one atom of carbonate of lime is chemically united with one atom of magnesia. 

 Even in the crystalline varieties we may often observe a small quantity of uncombined earthy 

 matter and other impurities ; and in the more ordinary forms of the deposit, carbonate of lime is 

 most frequently in considerable excess. In some cases, we may in great measure separate the 

 uncombined carbonate of lime by dilute acid, which does not so readily act upon the true dolo- 

 mitic portion of the rock ; and this accounts for the brisk eftorvescence of certain varieties of 

 magnesian limestone when first plunged in acids. There are also some rare varieties of the rock 

 (when its external character is both earthy and crystalline) in which carbonate of magnesia is in 

 excess. Of twelve specimens of dolomite examined by Tennant, ten were composed of one atom 

 of carbonate of lime, and one atom of carbonate of magnesia, with a small quantity of carbonate 

 of lime in excess. Two were of similar composition, but with a very small quantity of carbonate 

 of magnesia in excess ; and the specimen in which this excess was greatest, was derived from the 

 magnesian limestone formation near Doncasterf. We may, however, presume, that in all spe- 

 cimens which are highly crystalline, which effervesce feebly with acids, and are not much mixed 

 with extraneous matter, the two earths are chemically combined, and that the composition cf such 

 specimens will be found to agree very nearly with theory. To bring this to the test, I selected 

 from a very large number of specimens of dolomites, derived from various parts of the formation, 

 some beautiful and perfectly crystalline fragments ; and after an examination of some of them, 

 which was kindly undertaken by Professor Gumming, the result was what had been anticipated. 

 For example, one of the most crystalline rocks from Roach Abbey was found to be a definite 

 compound of one atom of carbonate of lime, and one atom of carbonate of magnesia, subducting 

 something less than two per cent of oxyd of iron and other impurities |. 



* From a hundred grains of this compact rock, Mr. Holme obtained the following result : — 

 Lime, 34.75, Magnesia, 16.125. Carbonic acid, 45. Black oxyd of iron and alumina, 1.373. 

 Silica, 1.25. Water and loss, 1.5. 



t See Phil. Trans. 1799, p. 314. 



+ The mode of examining the above specimens was as follows : — " Fifty grains were dissolved 

 in nitric acid, dried and redissolved in acetic acid; afterwards being converted into a mixed 

 sulphate and submitted to a red heat, the weight was 67.95 grains. This was lixiviated with a 

 saturated solution of sulphate of lime to separate the magnesia, and left a residue of sulphate of 



