ORIGIN" AND DEVELOPMENT OF ORE DEPOSITS. 117 



subsequent infiltrations. Still, for practical purposes, the above 

 classification will be found very useful, and probably at least as 

 accurate as any other that could be devised. 



Scope of the present work. As what may be called contempor- 

 aneous mineral deposits are somewhat rare and of little relative 

 importance in the West of England, and as even those which do 

 occur appear to have been subject to such an amount of change 

 since their first formation as to bring them fairly within the 

 scope of the third chapter of this essay, we need not consider 

 them further in sketching out the divisions of its subject, which 

 will be treated in five chapters, as follows : — 



Chap. I. — Formation of structural planes in rock-masses. 



Chap. II. — The mechanical phenomena of faulting. 



Chap. III. — Rock-change as affecting the formation of 

 ore-deposits. 



Chap. IV. — The formation and natural concentration of 

 detrital deposits. 



Chap. V. — The relative ages of the ore-deposits of the 

 West of England. 



In considering these various branches of the subject, we 

 shall begin with the rock-masses as already formed, or, at least, 

 in the first stage of their consolidation ; any detailed discussion 

 of the modes in which their materials have been accumulated 

 not falling within our plan. 



Chap. I. — Formation op Structural Planes. 



Structural planes in rock-masses are formed in drying, in 

 the process of solidification, as the result of molecular changes, 

 by mechanical means, &c, &c. They are either actual, comprising 

 the various " joint-structures," due mainly to shrinkage or strain, 

 or potential, comprising lamination, slaty-cleavage, quarry cleav- 

 age, and foliation, due partly to strain and partly to molecular 

 movements within the mass. 



Sec. 1. — Joint structure. In almost all rock-masses, whether 

 aqueous, igneous, or mytamorphic, divisional planes or joints 

 abound. These are of iwro classes, the first including main or 



