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solidation in the case of sedimentary rocks, and after their solidification 

 in the case of igneous rocks. 



Now the deformation of a rock-mass can only be produced by a 

 relative movement of some or all of its parts. Two extreme cases may 

 arise. The strain may be uniform throughout a considerable mass, in 

 which case the microscopic character of the rocks will be altered ; or 

 the strain may be localized along special planes, in which case the 

 parts enclosed between these different planes will retain their original 

 characters. The first is a case of what may be termed plastic deformation, 

 though it must not be supposed that the minerals of which rocks are 

 composed are plastic in the sense that they can change their form 

 without losing their individuality ; the second is a case of what may 

 be termed deformation by faulting. At first sight it appears as if these 

 two types of deformation were radically distinct. This, however, is not 

 so. There is in nature a most perfect gradation between plastic 

 deformation and deformation by faulting. 



In the sedimentary and igneous rocks which constitute the low-lying 

 districts on the outskirts of a mountain-range we find deformation by 

 faulting or gentle folding. The masses of rock which are enclosed 

 between the fault-planes have moved as rigid bodies The differential 

 movement is confined to the immediate neighbourhood of the fault- 

 planes, and so also is the dynamic metamorphism. As the mountain 

 range is approached the folds become sharper, the faults more numerous, 

 and the size of those masses which have moved as rigid bodies becomes 

 less and less. We observe, moreover, that the differential movement is 

 often no longer confined to the fault-planes but affects the rocks to a 

 greater or less distance from these planes. In such cases there is an 

 increase in the amount of metamorphism (crushing of the constituents, 

 development of new minerals, addition and subtraction of certain con- 

 stituents) as the fault-plane is approached, and we are led to the 

 important conclusion that the amount of metamorphism is in proportion 

 to the deformation. Continuing our imaginary course in the same 

 direction we notice that the folds become smaller and smaller, and the 

 faults nearer and nearer together. Hand specimens will now illustrate 

 the phenomena of folding and faulting as clearly as did large tracts of 

 country in the outlying districts. At last both folding and faulting sink 

 to microscopic dimensions, and it becomes impossible to discover, even 

 with the aid of the microscope, a cubic millimeter of rock which 

 has not been strained. Dynamic metamorphism which was localized at 

 the fault-planes in the outlying districts has now become general we 

 are in an area of regional metamorphism. Argillaceous rocks have been 

 converted into slates, phyllites, andalusite-schists ; sandstones into quartz- 

 ites and quartz-schists ; felspathic sandstones into mica-schists ; basic 

 igneous rocks into hornblendic, actinolitic, and chloritic schists ; acid 

 igneous rocks into porphyroids, gneisses, and sometimes even into mica- 

 schists. On reaching the central axis of the mountain range it is 

 found to be composed of coarsely crystalline rocks of a plutonic 



