PHYSIOGRAPHIC RELATIONS OF SERPENTINE 587 



sory epidote, quartz, iron oxide, etc., ranges approximately, 

 according to Van Hise,^ from 10 to 25 per cent. We are not 

 restricted here to bodies of pure or approximately pure chlorite, such 

 as chlorite schist; but may properly take account, also, of the vastly 

 more extensive chloritic rocks known collectively as greenstones 

 — massive, basic, igneous rocks (diabase, diorite, gabbro, etc.) — • 

 which have, in the zone of katamorphism, undergone the greenstone 

 alteration, of which chloritization is the principal phase. The 

 outcrops of dikes, stocks, etc., of greenstone are commonly pro- 

 tuberant; and if more salient than the relatively resistant nature of 

 the rock would lead us to expect, we may find in the expansion, of 

 which the chloritization is a reliable index, an entirely adequate 

 explanation. Furthermore, the differential movement resulting 

 from the expansion is very generally attested by internal 

 slickensides. 



Again, if the question be raised as to whether statenliths are 

 necessarily limited to silicate rocks, the answer, on theoretic 

 grounds at least, must be in the negative; for the hydration of 

 anh3^drite to form gypsum involves, in the absence of solution, the 

 very notable expansion of 60 . 3 per cent; and the resultant deforma- 

 tion, it is well known, may be very severe. Also, it is matter of 

 common knowledge that gypsum, in spite of its extreme softness 

 and ready solubility, is not infrequently characterized by decided 

 relief, and may be classed as to some extent a hill-forming rock. 

 Occurring mainly or normally as a sedimentary deposit, it must, 

 however, lack the power of persistent topographic rejuvenation, 

 save where the strata have exceptional thickness or are highly 

 inclined. 



The dynamic and structural relations of the statenliths to the 

 various other types of relief are expressed in the following sys- 

 tematic outline: 



Outline of a Genetic Classification of Reliefs 



Superficial Agencies 



Destruction (erosion) reliefs 



Youth = continuous, lobate plateaus 

 Maturity = connected, dendritic ridges 

 Senility = isolated hills — monadnocks 



^ U.S. Geol. Survey, Monograph 4^. 



