76 SCIENTIFIC AGRICULTURE. 



A Fault in a rock is the dislocation of strata, so that their 

 continuity is destroyed, and a series of strata on one or both 

 sides of the fracture are forced from their original position, 

 and raised one above another, or moved laterally. Faults are 

 generally filled with clay, sand and fragments of other rocks. 



A Gorge is a wide and open fissure or fault: when still 

 wider, with sloping sides and rounded at the bottom, it is 

 called a valley. 



Fig. 5. Dyke. 



A Dyke is a mass or wall of 

 rock interposed between the 

 ends of a dislocation, so as to 

 break their continutity: dykes 

 rarely send off branches. 



Veins are portions of rocks smaller than dykes, proceeding 

 from some large mass, and ramifying through a rock of a 

 different kind. Metallic veins were originally melted metals, 

 which were injected into the fissures and crevices of rocks by 

 some subterranean force. 



Fossil. This term includes those petrified remains of plants 

 and animals which are found in alluvium, or imbedded in solid 

 rock, and constituting part of its structure. 



Formations. The term formation is used to designate a 

 group of rocks having some character in common, either in 

 relation to age, origin or composition. JSvery formation con- 

 sists of several varieties of rock, all agreeing in certain qualities, 

 and occupying such relative situations as to indicate that they 

 were formed during the same period and under similar circum- 

 stances. Thus we speak of graywacke formation, gneiss for- 

 mation, coal formation, &c. 



CLASSIFICATION OF ROCKS. 



Many different classifications of rocks have been proposed, 

 none of which is entirely unexceptionable : the present state of 

 Geological science will admit of our adopting any one of them* 



