132 DESCRIPTION OF THE STRATA. 



the nodules consist of solid masses of pyrites, with little or no lime- 

 stone. They do not always contain organic remains. It is scarcely 

 necessary to add, that alumine and sil ex enter into the composition of 

 these stones, as well as lime and pyrites, though the two last are the 

 principal ingredients. 



These nodules are commonly of a lenticular or discoidal shape, 

 like flattened globes. Some of them are nearly or entirely globular, 

 which is particularly the case with several of the smaller nodules, 

 from 4 or 5 inches to 8 or 10 inches in diameter ; the larger masses 

 being always flattened, many of them resembling millstones with thin 

 edges. Several of the flat nodules are oval, and others are of an 

 irregular shape ; but the circular form is by far the most predominant. 

 It is of more importance to observe, that whatever form the masses 

 may assume, their flat surfaces or planes are always parallel to the 

 surface of the general bed ; and where they run in rows or courses, 

 as is often the case, such courses preserve the same parallelism, being 

 horizontal where the strata are horizontal, and dipping where they 

 dip. This remark, which we formerly made concerning the nodules 

 of flint in the chalk, and nodules of limestone in the argil lo-calcareous 

 sandstone below the oolite, is equally applicable to the ironstone 

 nodules occurring in the shale, both in the alum hills and in the oolite 

 district. Even the globular masses in the alum shale are surrounded 

 or marked with circular zones, the plants of which are parallel to 

 the plane of the strata in which they are imbedded. 



In the quarrying of the schistus for the purpose of making alum, 

 the nodules are all thrown aside. When long exposed to the air and 

 the rains, they aa'e partially decomposed, the sulphur of the pyrites 

 evaporating, and the iron undergoing oxidation ; and hence they ex- 

 change their blue or brassy aspect for a red or ferruginous hue, and 

 losing their extreme hardness, are split into shivery fragments. It is 

 observable, however, that the nodules which roll about in great 



