504 TALCOSE SCHIST. 



important change. To make this understood, we shall introduce as a preliminary a paper 

 read by the Chemist of the Survey at the meeting of the American Association for the 

 advancement of Science, at the Springfield Meeting, " On the so-called talcose schist of 

 Vermont," in 1859. 



ON THE SO-CALLED TALCOSE SCHIST OF VERMONT. 



The geological surveys of the various States, have made known to us the existence of a broad belt of 

 rocks, extending from Canada to Alabama, consisting of green schists, associated with gneiss and gneissoid 

 rocks. The green schists have been denominated talcose, both in New England and further south. In 

 our remarks, however, we would not be understood to affirm a non-magnesian character to any of these 

 rocks south of Massachusetts, because we have not examined them. At the same time there is a strong 

 probability that their mineralogical characters are the same; unless, perhaps, the disagreement existing as to 

 their age be full proof of their difference in composition. For the Canada Survey presume them to be 

 metamorphic rocks of lower Silurian age, while the Pennsylvania Survey regard them as hypozoic. First 

 of all observers in this country to our knowledge, Mr. T. S. Hunt, of Montreal, has stated that certain rocks 

 of this belt at its northern prolongation, which would be regarded by most observers as a highly talcose 

 schist, are almost destitute of magnesia, alumina being present in a large per cent, in its stead. An 

 analysis of one of these schists from St. Marie gave 



Silica, .... 66.70 



Alumina, . . . 16.20 



Peroxyd of iron, . . 6.90 



Lime, 67 



Magnesia, . . . 2.75 



Alkalies (by difference), . 3.68 



Water, .... 3.10 



100.00 



Upon the Chaudiere River, near Quebec, Mr. Hunt found specimens of pholerite in the crevices of the 

 ledges, whose composition was about forty-six per cent, silica, thirty-eight per cent, alumina, and 

 fourteen per cent, of water. To pholerite and pyrophyllite, both hydrated silicates of alumina and quartz, 

 Mr. Hunt has referred the composition of the so-called talcose schists. 



The same gentleman found that the clay slates or roofing slates in the vicinity of these talcose schists 

 had essentially the same composition as the latter, excepting that they contained a larger percentage of 

 alkalies, but never more than seven or eight per cent; the talcose schist that he analyzed giving nearly 

 three per cent, by difference. Hence he supposes that the talcose schists are formed from the clay slates by 

 metamorphism. 



He supposes that the mineral pholerite has been in a state of solution, "produced during the decomposi- 

 tion of the clay slates, which are made up to a large extent of the veins of feldspathic rocks. These slates 

 are slowly giving up their alkalies to infiltrating waters, and are thus being converted into kaolin. He adds, 

 " A great portion of the talcose slates of the Alleghany range, especially those associated with the gold 

 deposits, throughout the eastern part of North America, are derived from the alteration of clay slates, and 

 must be aluminous in their composition. It will be well for the future, to distinguish them, on account of 

 their luster, by the name of nacreous slates or nacreous schists." 



These observations induced us to examine the same rocks, as they occur in Vermont, in a more highly 

 metamorphic condition, and to ascertain whether their character is magnesian or aluminous. We selected 

 specimens from four different localities ; such specimens as were apparently the best characterized talcose 

 schists in the State. The results agree with those of Mr. Hunt. The analyses were kindly performed by 

 Mr. Gr. F. Barker, B. Ph., of Boston, a careful chemist, who has given the following account of them : 



