86 The Paragenetic Relations of Minerals. 



view of the origin of conglomerates generally, and likewise 

 of some porphyritic masses. 



The formation of ice upon ploughed land, and at the bot- 

 toms of rivers, appears to furnish a very instructive illustra- 

 tion of the mode in which conglomerates are produced. 

 When after long-continued rain a frost sets in, ice is rapidly 

 formed between the lumps of earth which are thus gradually 

 separated from each other. The formation of ground ice in 

 rivers commences in a similar manner between the pebbles, 

 and in both cases a kind of conglomerate is produced in 

 which the cementing substance is ice. At the present time 

 a conglomerate is gradually forming in the bed of the 

 Neckar in a precisely analogous manner. The water of this 

 river contains carbonates of magnesia and lime in solution, 

 and these substances are deposited in the form of dolomite 

 between the pebbles, separating them from each other, and 

 cementing them together. 



Immediately above the brown coal at Klein Augesd, near 

 Teflitz, there is a bed of quartz pebbles cemented together 

 by iron pyrites. There can be no doubt that this bed is 

 more recent than the underlying coal, and that the iron 

 pyrites is more recent than the overlying bed of clay. The 

 pyrites has here been formed by the reducing action of the 

 coal upon ferruginous solutions filtering through the clay, 

 and being deposited between the quartz pebbles, has gradu- 

 ally pushed them apart, and cemented the whole into one 

 mass. 



In the neighbourhood of Freiberg, a sandstone is now be- 

 ing formed from the sandy refuse of the ore washing. This 

 refuse contains iron pyrites, and the oxide of iron resulting 

 from its decomposition cements together the siliceous par- 

 ticles in such a way that hand specimens of the mass cannot 

 be distinguished from an ordinary ferruginous sandstone. 



In the alluvium of Meronitz (Bohemia), iron pyrites has 

 been deposited between fragments of pyrope, forming a con- 

 glomerate on a small scale. The cementing matter of the 

 pyrope is sometimes green opal, and more rarely gypsum. 



Judging from the phenomena of imbedded nodules, it ap- 

 pears, as Fournet has very justly remarked, that many stra- 



