Mr. Horner on the Geology of the Environs of Bonn. 449 



mixed with earthy lignite, is in many places very pyritous, and when that is 

 the case, works have been estabhshed for the manufacture of alum, as at 

 Friesdorf on the left bank, and Holtorf on the right bank of the Rhine, both 

 near Bonn. The clay, intermixed with earthy lignite, which also contains 

 pyrites, is piled up in lumps, forming heaps of many yards in length ; and 

 after being for some time exposed to air and moisture, decomposition takes 

 place, evolving much heat; sulphate of iron and sulphuric acid are formed, 

 which last unites with the alumine of the clay, and by repeated solutions and 

 evaporations, and the addition of potash, crystallized alum is obtained in the 

 usual way. This spontaneous heating, often extending to inflammation, some- 

 times takes place in the pits where the brown coal is worked. Between the 

 layers of the alum slate crystals of gypsum are often found, and large detached 

 crystals of gypsum occur in the clay, as in the London clay, in both cases 

 most probably produced by the decomposition of pyrites. 



Clay ironstone is of frequent occurrence in the clay, both in the form of 

 layers and in detached lenticular and irregular masses. In the neighbourhood 

 of Rott thirteen different layers were found, varying from an inch to a foot 

 in thickness, and making together nine and a half feet of ironstone*. Mas- 

 sive portions of crystalline spathose iron have been found at Scheurn near 

 Rott. 



C The Brown-Coal or Lignite. 



This is found in various states. 



a. A dark brown or black earthy substance, friable, sometimes pulveru- 

 lent, generally occupying the upper part of the lignite beds, and 

 rarely showing a stratified structure. 



6. A concreted mass, in which fragments of wood and leaves are visible, 

 with a mixture of earthy matter. 



c. Wood in different stages of bituminization, of all shades of colour, 



from light brown to black, and the latter sometimes approaching 

 to the nature of jet. 



d. Papierkohle, highly bituminous, burning with a bright flame, leaving 



a white ash, separating readily into leaves as thin as writing-paper; 



a deposit of finely comminuted vegetable matter and earth. 



The wood is generally in fragments of inconsiderable size ; but sometimes 



large stems of trees are found, and if lying horizontal, as they usually are, very 



often flattened. Trees have been met with in an upright position with their 



roots attached, and the stem passing through different beds, into which the 



* Noggerath, Das Gebirge in Rheinland-Westphalen, iv. 388. 



