1864.] T. STERRY HUNT ON LITHOLOGT. 187 



the chemical changes involved in the production of silicated min- 

 erals by the second method. In this way Delesse has shown that 

 in several cases where the chalk of Ireland has been altered by the 

 proximity of intrusive traps, the sand and clay which the former 

 contain have bei'n converted into calcareous silicates. (Ann. des 

 Mines [5], xii, pp. 189. 208, 212.) 



An instructive example 'f this process is furnished at Montreal, 

 where the bluish fossiliferous limestone of the Trenton group is 

 traversed by dykes of dolerite, which are subordinate to the great 

 intrusive mass of Mount Royal. The limestone for a iiistance of 

 a foot or two, is hardened, but retains its bluish tint. Within a 

 few inches, it is changed to a greenish-white color, which is seen 

 to be due to a granular mineral disseminated in the white car- 

 bonate of lime. The unaltered limestone- from the vicinity 

 contain variabl ' amounts of insoluble argillaceous matteis. A speci- 

 men treated with dilute hydrochloric acid, left a residue of about 

 twelve per cent of a fine clayey substance, colored by a SMiall 

 amount of carbonaceous matter, and mixed with a little pyrites, 

 which was removed by cilute nitric acid. This residue, after 

 ignition, gave to a solution of carbonate of soda, 9 5 per cent of 

 its weight of soluble silica; and the insoluble portion, being sub- 

 mitted to analysis, gave the result I. A portion of the limestone 

 which was near to the intrusive rock, andh;id become hardened and 

 partially altered, was subjected to the action of dilute nitric acid, 

 and gave an insoluble residue with the composition i[ The more 

 thoroughly altered greenish limestone was also treated with dilute 

 nitiic acid, which dissohed the carbonate of lime, and left a r si- 

 due, the analyses of which, from two diflferent portions of the rock, 

 are given under iii and iv. 



I. II. III. IV. 



Silica, 73.02 54.00 42.60 40 20 



Alumina, , 18.31 14.00 13.70 930 



Lime, 93 16.24 3169 36.40 



Magnesia, 87 5. 27 4 17 3.70 



Protoxyd of iron, traces 3.60 4-68 5.22 



Potash 5.55 3 14 undet. undet. 



Soda,. 89 1.22 " " 



Volatile, .90 1.20 1.20 



99 57 98.77 98.04 95.02 



The residue from the unaltered limestone, including the silica 



soluble in alkalies, contains nearly 75.5 hundredths of silica, and 



