BARKER'S ANALYSES. 505 



" Talcose Schist (so-called), Roxbury. This rock is schistose, friable, of a greenish-gray color, fissile ; sp. 

 gr. 2.72. The analysis gives the following result : 



Silica, 69.90 



Alumina, and peroxyd of iron, 20.00 



Lime, 1.51 



Magnesia, .... 1.80 



Soda, 2.33 



Potassa, 1.45 



Loss by ignition, .... 2.40 



99.39 



" From the analysis it will appear evident, that the name talcose applied to this rock is a misnomer ; 

 there can be neither talc nor chlorite in them, for both these contain from thirty-two to thirty-six per cent, 

 of magnesia. Mr. Hunt has proposed the name of nacreous slates, from their luster ; the small proportion 

 of water in this rock would seem to indicate some other mineral than pyrophyllite, and the total absence of 

 any exfoliation under the blowpipe would tend to confirm this impression." 



We would say of the position of the specimen analyzed, that it is from the middle of the talcose schist 

 formation, and is from the vicinity of a bed of verd-antique serpentine, which is decidedly magnesian in its 

 character. We selected this specimen as the most likely of any in the whole State to contain magnesia. 



" Talcose Schist, from Pownal. This is another schist of the same general character as the preceding. 

 Its color is bluish-gray ; not as friable, but its feel is more unctuous. In dissolving the fusion with 

 carbonate of soda in hydrochloric acid, hydrosulphuric gas was evolved, which blackened a solution of 

 acetate of lead ; yet the mineral, when treated with strong nitric acid and boiled, gave no reaction for 

 sulphuric acid, as would be the case had there been sulphurets present. The analysis was as follows : 



Oxygen. 



Silica, 42.90 22.73 



Alumina, and peroxyd of iron, 42.20 19.71 



Lime, 78 .22 



Magnesia, . . . . 1.98 .79 



Potassa, 5.24 .89 7.22 



Soda, 1.33 .34 



Loss by ignition, . . . 5.60 4.98 



100.03 



That this is a distinct rock from the last, is shown by the diiferences in the silica and alumina, there 

 being twenty-seven per cent, less of silica, and twenty-two per cent, more of alumina. The specific gravity 

 is greater, being 2.90 ; the magnesia remains nearly the same, while the alkalies and water have doubled. 

 An analysis of talcite (Dana's Mineralogy, 4th ed. p. 509), by Tennant, strikingly agrees with this in 

 composition. The specimen used was from Wicklow, Ireland (Cambrian.) The oxygen ratios in this 



specimen will be seen to be nearly 1:3:3 (that is 1 : 2.7 : 3.14) giving the formula (RO, R 2 3 ) Si.0 3 



The specimen analyzed is an ordinary sample of a somewhat decomposing schist, which is called 

 magnesian slate by Prof. Emmons. It is one of the members of the Taconic System, which in this Report we 

 describe as talcoid schist. Pownal is upon the classic ground of this system of rocks, and there can be 

 no doubt that this specimen was taken from the true magnesian slate of that system. 



" Talcose Schist, Middlesex. This is another of the so-called talcose schists, perhaps more properly 

 talcoid. It is less schistose and more slaty than the preceding specimens ; color olive-green ; odor argil- 

 laceous : composition, 



Silica, . . . f . . 64.10 



Alumina and peroxyd of iron, 23.50 



Lime, .."... .84 



Magnesia, .... 1.98 



Potassa, 3.70 



Soda, .... 2.20 



Loss by ignition, . . . 3.60 



99.92 

 33 



