316 DYNAMICAL GEOLOGY. 



3. The carbonic acid of limestones is driven off at a low temperature, as 

 in limekilns. But under heavy rock-pressure the loss does not take place ; for 

 limestone may be melted in a strong iron flask without decomposition, as 

 shown by Sir James Hall (1790). Again, when iron carbonate (FeO. 00^) 

 is present in a sandstone, heat may expel the carbonic acid (CO2) and 

 leave the iron to oxidize and become the red oxide (FeaOg). This is a second 

 source of the red color of red sandstones and shales. But under pressure 

 the ore may be crystallized without loss. 



4. Consolidation of rocks also goes forward in the feebler stages of 

 metamorphism. Subjection to heavy superincumbent pressure forces the 

 particles into closer contact, and this favors consolidation in clays (W. 

 Spring). The consolidation in the case of ordinary shales, even Silurian, as 

 the Utica shale, is feeble, unless some metamorphic heat has given aid. 



2. Crystallinic Metamorphism. 



Calcyte (CaO. CO2), or dolomyte, magnesian limestone, if pure, becomes 

 under metamorphic action a white crystalline rock, like architectural or 

 statuary marble, in which state, as the naked eye may detect, each grain has 

 the cleavage of crystallized calcite or dolomite. The process is simply that 

 of crystallization. It is a change without fusion. It is a molecular change 

 solely, like the change which takes place in tempering steel from fine 

 to coarse, or the reverse. 



Again, under slow metamorphic action, a granitic sandstone, consisting of 

 quartz, feldspar, and mica (the constituents of granite), loses the worn surfaces 

 of the grains and becomes a granite. The sandstone being a massive rock, it 

 is massive still — a true granite, and not gneiss. A sandstone, consisting of 

 feldspar and quartz, without the mica, becomes the granite-like rock called 

 granulyte. Such sandstones make up the Triassic of the Connecticut Valley, 



and some portions, well consolidated, look exceed- 

 ingly like granite, although they have not been 

 subjected to the heat and pressure of the true 

 metamorphic process. The following analysis, by 

 F. W. Taylor, of the Connecticut rock, from Port- 

 land, — the common building stone, — shows its 

 granite-like composition (see 10th Census, Vol. 10, 

 Rep. on Building Stones, page 127) : silica 69-94, 

 alumina 13-55, FeA2-48, Mn2O3 0-70, lime 3-09, 

 soda 5-43, potash 3-30, moisture 1-01 = 99-50. If 

 the granitic sandstone were thin-bedded it might 

 become gneiss ; and a shale might make a mica 

 Grain of quartz of Potsdam sand- schist of like composition. Morcover, in the in- 

 TcTyZ^Tl'ilTs^T tenser stage of metamorphism a bedded_ granitic 



sandstone, instead of being changed to gneiss might 

 become plastic or fused, and so lose all bedding and become granite. Such 



