of the Minerals of the North of Ireland. 19*7 



lied. That analysed by Da Costa occurs wherever the trap 

 dykes cut the chalk*. Dr. M'Donnell, who first noticed this 

 mineral, informs me, that it was pronounced to be dolomite by 

 several skilful mineralogists and chemists to whom he showed 

 it at various times. The analysis of Da Costa, however, gave 

 no magnesia. The other mineral yielded to Dr. Thomson 

 some portion of that earth. It has been found only at Down- 

 hill in Derry, in veins and irregular masses in an amygdaloid 

 of a loose texture, accompanied by zeolites and the common 

 carbonate of lime. On these grounds I think the two minerals 

 may be regarded as distinct; an analysis of the magnesian one 

 will most probably be given in full in the work on mineralogy 

 for some time expected from the pen of the distinguished 

 chemist just named. In the System of Professor Mohs, and 

 the excellent "Manual," by Robert Allan, Esq., just published, 

 this and several others, usually described as distinct, are 

 classed under calcareous sparf. 



It was stated in the " Catalogue," that the large crystals of 

 quartz so frequent at Dungiven were found in a trap rock. 

 I believe this is inaccurate. Some of the crystals are found in 

 the bed of the river Roe, which is partly primitive ; others are 

 found in the debris of mountains of the basaltic range, but 

 have most probably been transported thither by currents from 

 the primitive country to the west. Small crystals of quartz, 

 however, are frequent in our trap rocks. Hydrolite and 

 levyne have been recently found in other places within the 

 basaltic district besides Island Magee, and anhydrous disili- 

 cate of iron in dykes and loose boulders of trap near Larne. 

 Mr. M'Adam suggests that it ought to have been mentioned 

 in the " Catalogue," as a remarkable circumstance, that the 

 magnetic or octahedral iron ore found by him in the Isle of 

 Muck, occurs only on the external surface of the rock. 



The researches of Lieut. James, R.E., in the county of 

 Down, and the zealous labours of Mr. Patrick Doran in all 



* It perhaps ought to be observed, that this mineral may have been 

 found in the vicinity of the Causeway, but certainly not immediately there ; 

 because there is no chalk at the Causeway, nor, so far as I am aware, at 

 any place in the trap district where columnar basalt exists. In such a case 

 that rock rests on lias or sandstone. 



f [We are obliged to Mr. Bryce for his reply to our inquiry. We do 

 not know what edition of Prof. Mohs's System he may refer to, but in 

 Haidinger's Translation neither of the minerals in question is adverted to, 

 either by name or by implication ; nor would it be in accordance, we think, 

 with Prof. Mohs's principles of classification, to include them ** under 

 calcareous spar " : the other minerals " usually described as distinct " but 

 which are also regarded by that mineralogist as varieties of calcareous spar, 

 consist, essentially, like that mineral, of carbonate of lime alone, and are 

 also connected with it into one species by gradual transitions of external 

 character, or by what in Zoology and Botany is termed affinity— E. W. B ] 



