222 MINERALOGY AND LITHOLOGY. 



Dr. Jackson describes one such broken mass of slate in Gilford, in 

 which the strata were likewise entirely broken to pieces. The effect was 

 attributed to an immense iceberg which was stranded there while the 

 land was submerged, and by its movements and its mighty weight broke 

 up the rocks. Although such accidental causes might produce these and 

 many other results, in the more general and efficient action of the great 

 movements that have taken place in the crust of the earth, a more relia- 

 ble explanation is found for all such varied and often peculiar effects. 



Novacnlite. (Oil Stone.) This is a variety of schist in which the 

 quartz very largely predominates, and the rock therefore loses its schist- 

 ose character, becomes nearly massive, and breaks with a conchoidal 

 fracture. It is usually of a gray color, and forms layers of greater or less 

 thickness among the other schists. These rocks have not been well 

 understood, and have been called quartzite, siliceous limestone, and 

 felsite ; but the microscope shows that they possess the same structure, 

 and the same ingredients arranged in the same way, as the argillitic mica 

 schists, and that the proportion of ingredients is all that constitutes a 

 difference. 



As tpyical of these rocks, a specimen from Littleton will be described. 

 This rock is light gray in color, massive, and so fine in its texture and so 

 homogeneous, that no ingredient can be macroscopically detected. It 

 looks like a gray felsite, and like felsite it fuses before the blow-pipe. A 

 study of a thin section shows that it is an excessively fine-gramed mixt- 

 ure of much quartz and little orthoclase, among which the little fibrous 

 and scaly crystals of mica that characterize the argillitic mica schists are 

 thickly scattered ; but these scales are much smaller, and do not consti- 

 tute an ingredient of any importance. Grains of calcite are also seen. 

 The constituent minerals bear a recrystallized character, and none of 

 them appear fragmental. The rock has, therefore, all the characters of 

 argillitic mica schist; and the massive condition is due to the excessive 

 amount and fineness of the quartz, and the small amount of the mica- 

 ceous constituent. Its fusibility, which first caused it to be called felsite, 

 is in part due to the calcite, which forms a flux for the silica, and in part 

 to the orthoclase, for it is well known that a mixture of quartz and ortho- 

 clase will fuse as easily as orthoclase alone. A specimen from Tamworth 

 is black, and contains chlorite and iron oxide. Other specimens differ in 



