522 



PHYSICAL GEOGRAPHY. 



the ferruginous appearance results from the decomposition of iron 

 pyrites, it is perhaps reasonable to look to the waters of the ocean for 

 the small percentage of sulphur required to combine with the iron of the 

 mud. This may have been reduced from sulphates in the water through 

 the agency of organic matter. The andalusite rocks were probably very 

 clayey, deficient in alkali. This clay might result from the decomposition 

 of feldspar. As the removal of potash and soda from feldspar is the pre- 

 requisite for its disintegration, the resulting clays might be deficient in 

 the alkalies. After metamorphism we should therefore expect the pro- 

 duction of schists corresponding in mineral composition, or with the 

 predominance of silicates of alumina without an alkali, like andalusite, 

 fibrolite, staurolite, and kyanite. These schists do not usually contain a 

 large proportion of feldspar. 



The Concord granite would probably come from argillaceous sand- 

 stones, the silica being rather meagre in amount. This division has 

 probably been subjected to peculiar influences, so as to induce the com- 

 pact, even structure of the rock. The appearances would indicate a long 

 continued quiet metamorphic action, which has reduced the minerals to a 

 uniformly fine texture. The rock is evidently the equivalent of the 

 celebrated Monson, Mass., stone, where the layers of deposition are as 

 evident as in a freshly excavated hill of sand. It would seem as if the 

 action in the latter case had been retarded before the conversion of the 

 material into so homogeneous a mass. 



More than one period of metamorphism seems to be indicated by the 

 facts. The Franconia breccia holds in its embrace fragments of meta- 

 morphic rocks of all the varieties that are peculiar to the Atlantic sys- 

 tem in that part of the state. The original formations must therefore 

 have all been altered prior to the production of the breccia. Possibly 

 this rock belongs to the following period, but it was as certainly prior 

 to that, as it clearly followed the Atlantic age. Hence I have regarded 

 it as marking the transition between these two great eras. 



Use of the terms Atlantic and Montalban. 



Those who have read the annual reports will observe that this is the first occasion 

 in which I have used the term Atlantic. My first report, printed in 1869, proposes the 

 name of White Mountain series for all the gneissic rocks east of the staurolite rock in 

 Lisbon (see page 17 of this volume) . In 1870, Dr. T. Sterry Hunt wrote a letter to Prof. 



