140 METAMOEPHIC EOCKS 



mentioned, to a double decomposition and gradual replacement 

 of the calcium in the carbonate, by magnesium, the continual 

 percolation of sea-water with its normal content of magnesia 

 being sufficient to bring about the result. It was noted, however, 

 that in none of these cases did the per cent of magnesian car- 

 bonate quite reach that of true dolomite, 40 to 42% being the 

 maximum amount of this constituent found. 



Yet another form of change in the structure and mineral 

 composition of a rock is that brought about through the action 

 of water below the zone of oxidation and of true weathering. 

 It may be best described as a process of hydro-metamorphism, 

 since the influence of water is paramount. It is to this form 

 of metamorphism that is due the production of secondary epidote, 

 chlorite, sericite, leucoxene and various zeolitic compounds from 

 pre-existing minerals without in any way changing the character 

 as a geological body of the rock mass in which they occur. Such 

 changes are in part metasomatic, and In many instances are 

 rendered more intense by dynamic causes. This form of change 

 has, unfortunately, been too frequently confounded with wea- 



Under the head of metamorphic, then, is grouped a large 

 series of rocks which have been changed from their original 

 condition through the dynamical and chemical agencies above 

 described, and which may have been in part of aqueous and in 

 part of eruptive origin. Were it possible, it might have been 

 better to describe each class of these rocks together with the 

 corresponding igneous or aqueous form from which it was de- 

 rived by this process of change. In only too many cases, how- 

 ever, the metamorphism has been so complete as to quite obliter- 

 ate all such traces of the original character as would lead to safe 

 and satisfactory conclusions, and consistency demands that all 

 be grouped together. 



Accordingly as they vary in structure the metamorphic rocks 



1 WTiile it is true tliat no new compound can be formed without first a 

 breaking up, or decomposition, of those already existing, still, as this de- 

 composition affects only the individual minerals, and not the integrity of 

 the rock mass as a whole, it would seem preferable tc include such changes 

 under the name of alteration and metamorphism. Weathering it certainly 

 is not, though it is essentially the form of change which Eoth (Allegemeine 

 u. Chemische Geologic, Vol. I, pp. 159-412) has designated as complex 

 weathering (Complimrte V erwitierung) , See also A Discussion of the Use 

 of the Terms Eockweathering, Serpentinization and Hydrometamorphism, 

 Geological Magazine, London, Aug., 1899, and American Geologist, Oct., 1899. 



