TALCOSE SCHIST. 507 



schists in the northern part of Vermont is several times as wide as the same formation in Southern Vermont ; 

 but by including two belts of gneiss with the talcose schists in the southern part, the whole width corres- 

 ponds to that in the northern part. In southern Massachusetts, the same belt of talcose schist entirely 

 disappears, to be replaced by gneiss; but again in Connecticut, we believe, the talcose schist reappears. 

 Thus a rock disappears and reappears while the strata have the same dip and strike, and we can discover 

 no line of demarkation between them. By recurring to the analysis, we find that the composition of all 

 of them correspond with that of the gneiss also for the principal constituents are silica, alumina and 

 alkalies and the latter is as largely developed in the schist as in several analyses of gneiss that we have 

 consulted. Hence then we have another stage of inetamorphism : first there is a simple sediment if 

 coarse, it is sandstone ; if fine, clay slate. A change takes place called metamorphism the effect of heat 

 and water combined which converts these sediments into nacreous or aluminous schists. The continued 

 and probably intensified action of the same causes converts these schists into gneiss, or feldspathic rocks. And 

 were we to imagine a still greater increase of temperature, this gneiss might melt into granite thus the whole 

 process of the strongest metamorphosis would be seen. If this is so, it is possible that we have instances of 

 the last stage of the metamorphosis in occasional hills of granite in the talcose schist formation, as in the 

 towns of Bridgewater and Braintree in Vermont, and in the gneiss of the Green Mountains in the towns of 

 Stamford, Sunderland, Stratton, etc. The talcose schist formation which we have been describing lies upon 

 both sides of the principal range of the Green Mountains, especially upon the east side. But the Green Moun- 

 tain range itself in Massachusetts forming Hoosac range, and in Vermont extending through the State 

 is composed of gneiss and gneissoid rocks. In northern Vermont, in the range of Mansfield Mountain 

 and Jay Peak, it becomes more talcose, but with shining crystals of feldspar appearing in the edges of the 

 strata. We never should have regarded this rock in northern Vermont other than talcose did we not examine 

 the formation along its axis and notice the gradual change into talcose schist with crystals of feldspar. In 

 several localities in this vicinity this rock may be called a talcose sandstone, whose sedimentary character has 

 nearly disappeared in consequence of the metamorphic action. Thus in this principal range of the Green 

 Mountains may we see another example of the change of talcose schist to gneiss. 



We say talcose schist for two reasons : the universality of the non-magnesian character of this rock, 

 though probable, ought not to be considered as settled by five analyses. Almost every stratum of this 

 rock presents some variation from the adjacent ones. It may be that a large number of these may yet be 

 found to be magnesian, while many are aluminous. If so and it should appear at some future day, it would 

 be much better for the cause of science that we retain the old name, talcose, for the present, and at the same 

 time express the conviction that probably there is no magnesia present in any of it. Such a scientific con- 

 servatism is always safe. And, secondly, the analysis of some of these specimens shows that they agree 

 with the mineral talcite. Hence it will be perfectly proper to use the name talcite schist, or its congener, 

 talcose schist. For these two reasons, then, we think that it would be well to retain the old name for the 

 present. 



After discovering that magnesia in talcose schist was unfrequent, among our metamorphic rocks, the in- 

 quiry arose, how is it with talcose schist in other localities does it contain magnesia or not? Mr. Barker 

 referred to four analyses of talcose schist from Bischof (Bischof's Geology, Cavendish Soc. ed., p. 352), 

 three of which agree essentially with the Vermont specimens, but the fourth contained 26 parts of mag- 

 nesia. The non-magnesian specimens were from Sweden, Germany, and Tyrol ; the magnesian example 

 from Austria. 



Bischof adds this note : " What is often called talcose slate contains none or merely traces of magne- 

 sia, as analyses I, II, III, show." 



Thus there are two great varieties of talcose schist in Europe, one of them corresponding to the alumin- 

 ous schist in New England. Whether any true magnesian slate from this country has ever been analyzed, 

 we are unable to say. 



NOTE. In a letter from Mr. T. S. Hunt, written since the reading of the preceding article, several 

 suggestions are made which deserve to be mentioned, and therefore I add an extract from it : 



" I did not hear your paper on talcose schists, and do not know but you have compared your results of 



