32'0 M'..(H. } 



1864.] T. STERRY HUNT ON LITHOLOGY. 177 



niin^iled with grains of pyroxene, occasional plates of mica, and 

 grains of pyrites. It contains no carbonates. Two an.iiyses of 

 portions of the dolerite, from dykes differing- a little in texture, 

 gave as follows under XV and xvi : 



XV. XVI. XVII. 



Silica 50. 3n 50.25 52/20 



Alumina 17.35 



Peroxyd of iron 12.50 



Lime 10.19 9.63 7 34 



Magnesia 4.93 5.04 4 17 



Potash 69 .58 2.14 



Soda 2.28 2.12 2.41 



Volatile 75 1.00 2.50 



99.04 100.72 99.26 



The iron in these analyses, although given above as peroxyd 

 exists in the form of protoxyd, and in the second specimen, in p.nt as 

 a sulphuret. These rocks, which appear to have the compo.-sition 

 of m xtures of a basic feld.spir with pyroxene, do not differ from 

 ordinary dolerite. 



riie newer dolerite, which cuts the three other clas>cs of eruprive 

 rocks in the Laurentian region, has a grayish-black, very titie-»'r iued 

 bcise, earthy and sub-conciioidal in frtcture, atid reseuibiino' 

 somewhat the preceding. It cont.iins small brilliant black grains of 

 ilmcnite, with others of sphene, and small sc.iles of mica. Occa- 

 sional masses of black cleavable augite, sometimes half an incu in 

 diameter, give to the rock a porphyritic character. It contains 

 besides, small cleavable masses of white carbonate of lime, with 

 which the whole rock seems penetrated, When in powder, it 

 effervesces freely in the cold with dilute nitric acid, and the vsolu- 

 tion evolves red fumes on heating. In this way there were dis- 

 solved, lime, equal to 8.70 per cent of carbonate, 0.50 ol' n»agne- 

 sia, and 6.50 of alumina and oxyd of iron =r: 15.70 per cent. Tiie 

 residue dried at 211^ F , equalled 83.80 per cent. A poiiir)n of 

 aluminous silicate had evidently been attacked by the aciti. The 

 dried residue gave^on analysis the results which will be found above 

 tinder XVII. 



The dolerites of the Montreal district, besides forming iramer- 

 ous dykes, constitute the chief portions of the mountains of Mon- 



• With souae titauic acid. 

 Vol. I. u No. 3. 



