64? 2 Contributions to the 



ture ; it is slowly soluble in acids, so that, for its speedy solu- 

 tion, it is necessary to pulverise it and apply heat. Specific 

 gravity, 2*723 ; colour grey ; lustre dull, occasionally pearly 

 from the calcareous spar embedded in it ; opaque ; hardness, 

 3*5. Before the blowpipe, per se, becomes yellowish; on 

 charcoal, is rendered caustic ; fuses, with phosphate of soda 

 and borax, into a transparent bead ; and, with carbonate of 

 soda, into an opaque one. Its constituents are : — 



Carbonate of lime - - - 49.6 



Carbonate of magnesia - . - 44<* 



Silica - - _ _ - 4* 



Peroxide of iron - - - 1*2 



Alumina - - - - - !• 



99-8 



If we abstract the impurities, this corresponds very nearly with 

 Carbonate of lime - - - 6.25 



Carbonate of magnesia - - - 5-25 



11-5 



Hadden Rigg, which constitutes a rising ground on the 

 south side of the river, and runs parallel with it, consists of 

 this mineral and claystone porphyry. It may be observed 

 two miles higher up the Tweed ; and the sandstone quarry 

 of Sprouston seems to repose on it. There is an excellent 

 section of the mineral and its alternating strata along the bank 

 of Carham Burn, a small streamlet which separates the two 

 kingdoms, and which makes a conspicuous figure in the bor- 

 der history under the name of Reddenburn. It rushes with 

 considerable impetuosity down a declivity, in order to dis- 

 charge its waters into the Tweed, tearing up the soil and 

 rocks, and has exposed the strata for the inspection of the 

 geologist. Immediately under the soil lies a stratum of clay- 

 stone porphyry of a purplish colour, in which the clay is most 

 predominant, containing some nodules of quartz, &c. Then 

 comes a stratum of magnesian limestone ; with red hornstone, 

 a mineral which is very abundant here, often of a fine flesh 

 red colour, but sometimes almost as pale as quartz : and thus 

 a succession of alternations proceed, till we arrive at the bed 

 of the rivulet. Fig. 115. is a section and measurement of 

 these strata. 



The porphyry is sometimes very finely granular, having ra- 

 ther a beautiful appearance, and is so hard as to be used for 

 mending the roads. It seems to occupy the place of the brec- 

 cia, which occurs in connection with the magnesian limestone 

 on the coast of Durham ; and a similar rock is said to exist 

 along with the dolomite of the coal country of Upper Silesia. 



The same limestone lies under sandstone at Eccles, but is 



