54 M. Mobs' Summary of Geognostical Phenomena. 



formation exists. On the contrary, coal occurs in limestones, 

 in sandstones, and in some other rocks, and even in beds of 

 clay, sand, and loam, or covered by these, by basalt, and simi- 

 lar mineral matters ; and all the coals that are found under 

 these diversified circumstances are not of the same nature and 

 quality. Beds of coal make their presence more readily known 

 than beds of other useful minerals, by their outcrop ; but still, 

 it is seldom the case that their true nature is there exhibited ; 

 and it cannot, therefore, be ascertained at such points. 



Rock-salt occurs in irregular masses, which are surrounded 

 by limestone ; but it likewise forms beds, generally of great 

 extent, and irregular masses, in several other rocks, to which 

 different ages are assigned, especially in sandstones and the 

 newer limestones. Owing to its easy solubility, it is seldom 

 seen on the surface of rocks ; but, where it does so occur, it 

 presents itself in whole rocks, which, in their interior, possess 

 their natural characters, but on the surface have lost more or 

 less by being dissolved, but have not otherwise sustained any 

 important change. The rock of which the greater part of the 

 mass of the irregular repository included in the limestone con- 

 sists (Das Haselgebirge), is often exposed in the form of clay, 

 mixed with gypsum, &c, but under these circumstances it does 

 not always contain salt, which has either disappeared by solu- 

 tion, or has not originally been present ; for, as the coal-for- 

 mation is not unfrequently found without coal, the saliferous 

 strata often occur without salt. The presence of rock-salt is 

 sometimes made known by the salt springs which originate 

 from it, and these are worthy of attention, not only on that 

 account, but also because they themselves often form a subject 

 of investigation. 



The ironstones which occur in the rocks now under consi- 

 deration are chiefly brown and clay ironstones, including un- 

 der the latter the lenticular clay iron-ore, and the pea iron- 

 ore, none of which are confined to any particular rock, except 

 the pea iron-ore and the common clay iron-ore (an impure 

 variety of sparry iron, mixed with clay), of which the former 

 is generally found in cavities in limestone, and the latter gene- 

 rally in the coal-strata. Most of these ironstones betray their 



