382 MR. E. WETHERED OX THE DEVONIAN 



the constituents replacing a magnesian silicate. Thus, Prof. 

 Bonney and Mr. F. T. S. Houghton state, in their paper ' On 

 some Mica-Traps from the Kendal and Sedbergh Districts/^ that 

 they consider " much of the mineral in these mica-traps to be 

 dolomite." 



But is there evidence of magnesian silicates having been depo- 

 sited with the limestone ? When I come to describe the residues 

 left after boiling the limestone in hydrochloric acid, I shall show 

 that detrital minerals deposited with the limestone have under- 

 gone decomposition ; that being so, the bases would be liberated 

 in a soluble condition. The question then arises whether we have 

 evidence that magnesian silicates are or have been present in 

 the residues. Of the decomposition of silicates of some sort 

 there is proof, but not that any magnesian silicate other than biotite 

 has been represented. That micas of this group do not easily 

 undergo decomposition is well known, but it must be remembered 

 that some of the limestones of South Devon have been subjected 

 to influences which have greatly altered their normal condition ; I 

 mean, of course, that they have been rendered crystalline. It is 

 in this crystalline limestone that the dolomite crystals are most 

 numerous, and the detrital residue most decomposed. I must 

 moreover point out that, in searching for instances of decomposed 

 micas in the residues, the process of boiling in acid would tend to 

 remove the decomposed portions, leaving only the silica. The same 

 observation would apply to other silicates. 



It is worthy of remark that the rhombohedral crj^stals of dolomite 

 are mostly associated with calcareous organic fragments. This may, 

 perhaps, be explained by the greater solubility of the aragonite as 

 compared with the other form of carbonate of lime of which shells 

 are mostly constructed. 



§ 4. Residues insoluble in Hydrochloric Acid. 



The residues were obtained by boiling portions of limestone in 

 strong hydrochloric acid. The residues from this treatment were 

 allowed to subside in the boiling flask and the liquid was poured 

 off. Distilled water was then added and the flask shaken so that 

 the residue might be cleared of calcium chloride. This process 

 was repeated four or five times, the residue being allowed to subside 

 each time before the liquid was decanted. The calcium chloride 

 having been removed by this means, the finer portion of the residue 

 was poured ofl" into a watch-glass and portions were removed to 

 a glass slide. These were then dried over a small flame and 

 covered with Canada balsam and a cover-glass. The heavier portion 

 of the residue was mounted in the same way. 



The following table shows the percentages of residue obtained 

 from various typical examples of Devonian Limestone from South 

 Devon : — 



Quart. Journ. Geol. Soc. vol. xxxv. (1879) p. 167. 



