TALCOSE SCHIST. 503 



^N 



tion from solution in water has given rise to some varieties. Yet, perhaps Bischof would 

 say that the silex held in solution by Avater was the result of the decomposition of silicates. 

 In that sense his proposition might be true as to the origin of quartz. But we are en- 

 deavoring to point out its state immediately preceding that in which we now find it. 



3. By the abstraction of soluble silicates. We refer here to those cases where highly 

 silicious masses, such as pebbles, have been permeated by alkaline water, and this has 

 dissolved out and removed the silicates, leaving the excess of silica in the form of white 

 or hyaline quartz. We have given our reasons for believing that such an abstraction has 

 taken place in certain conglomerates, and the tertiary coal, and we strongly suspect that 

 it has extended to most of the cases where tubercular masses of quartz in the schists ex- 

 hibit great purity. 



4. By the conversion of limestone into quartz rock. The process is well described by 

 Bischof: " Supposing a bed of limestone, uniformly permeated by water, containing sil- 

 ica, during a long period, nothing is easier to conceive than that, by the removal of 

 carbonate of lime and the substitution of silica in its place, the whole rock would ulti- 

 mately be converted into a quartzose mass." This process might stop at any point of 

 the metamorphosis, and the result might be silicified rocks containing more or less of 

 carbonate of lime, which are often called silicious limestones. This case is well illustrated 

 by what is called the red rock in Vermont, which, near Burlington, produces the beautiful 

 Winooski marble. 



5. By the conversion of sandstones into quartz rock. If the sandstone consist of grains 

 of pure quartz, which is rarely the case, its permeation by water holding silex in solution 

 might furnish a silicious cement, so that the rock would become granular quartz. But if 

 the sandstone comes from rocks containing more or less of silicates, and these be pene- 

 trated by water with alkalies, the result would be an abstraction of the soluble silicates, 

 leaving the excess of silex as quartz. We are inclined to believe that this may be one of 

 the modes in which extensive quartz formations have originated. 



To account for the varied forms of quartz in the rocks of Vermont we think we must 

 call in most of the preceding modes of its production. Its most usual origin, however, is 

 as a residuum in the processes of metamorphism. Any attempt to derive it from igneous 

 fusion appears improbable, because of all substances it is one of the most infusible. 



TALCOSE SCHIST. 



We describe this rock as a formation including, besides the lithological talcose schist 

 and its varieties, the associated belts of rocks of different structure. The three ranges of 

 it combined cover perhaps a larger area of the State than any other group of rocks. 



We shall not include steatite or soapstone in this formation ; first, because it is not con- 

 fined to the talcose schist although most common here ; secondly, because it now appears 

 that there is scarcely any more affinity between steatite and talcose schist than between it 

 and mica schist or gneiss. 



Lithological Characters, with interstratified rocks and veins. 



Talcose schist proper consists of quartz and talc. But in consequence of the researches 

 of chemists our views respecting the composition of talcose schist have undergone an 



