METAMORPHISM. 705 



cation. The soft sandstones of all ages, from the Potsdam to the 

 incoherent beds of the Post-tertiary, are evidence on this point. 

 It is probable that deposits to an immense extent have existed in 

 past time that failed to be consolidated, and were consequently 

 washed away in the course of subsequent changes. 



But, while there are many fragile Potsdam sandstones, there are 

 others, as those of eastern New York and Vermont, that have been 

 hardened through the metamorphic process into quartzites or gra- 

 nular quartz rocks ; and deposits of sand and pebbles of various 

 other ages that are refractory sandstones and grits. That the con- 

 solidation took place through the metamorphic process, is often 

 evident from their position within, or on the outskirts of, regions 

 of other metamorphic rocks. In the same way, fragile absorbent 

 argillaceous shales have been hardened into firm non-absorbent 

 slates. 



The common modes of consolidation not here included among metamorphic 

 processes (although the term in its widest meaning might comprehend them) 

 are the following: — 



(a.) Carbonate of lime, derived from granulated shells or limestone, is often 

 disseminated through arenaceous beds, and, when so, infiltrating waters may 

 take into solution and deposit again some of the carbonate, and thus cement 

 the sands. Blocks from a soft calcareous sandstone often increase in hardness 

 after being removed from a quarry and put into a structure where they are 

 exposed, over the surface at least, to alternate drying and moistening through 

 atmospheric causes. 



(b.) Beds of sand often contain disseminated grains of some ore of iron, which 

 are altered by infiltrating waters, and which afterwards become solid and thus 

 solidify the mass. The decomposition of iron pyrites, and the peroxydation of 

 its iron, and of that in carbonate of iron and magnetic-iron ore, are the common 

 methods. The result is often a rock stained, or wholly colored, red or broionish- 

 yelloic, — the former color when the process goes on out of water where the atmo- 

 sphere has free access. The ferruginous material may also be derived from 

 external sources. 



2. Loss of water or other vaporizable ingredients. — The water contained 

 in the original material of a rock is sometimes wholly, and some- 

 times but partly, expelled. Serpentine is a metamorphic rock 

 containing 12 per cent, of water ; and talcose slate contains 5 or 6 

 per cent. In many others more crystalline, water is essentially 

 absent. The bitumen of bituminous coal has been partly or wholly 

 driven off by the process, and anthracite and semi-bituminous coal 

 formed (p. 410). 



Carbonic acid is expelled from carbonate of lime, or limestone, as 

 is well known, in a heated lime-kiln. But in the metamorphism of 

 limestone it is retained. It has been shown by experiment that 



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