GEOLOGY. 235 



veins of crystallized calcite in morlern coral reefs. Infiltration 

 of water holdino: carbonate of lime in solution will account for the 

 formation of calcareous sandstones, g-iits, and tuffs, and the fillinij^ 

 up of the pores of many vehicular r<K'ks and lavas with carbonate 

 of lime; the reverse effects would follow from such rocks bcin;]^ 

 subject to the action of waters which could dissolve out such 

 bodies previously existins^ in the pores of rocks. Certain zeolites 

 or hydrated silicates may have been formed by the water entering 

 into chemical combination with previously anhydrous silicates. 

 Many magnesian limestones and dolomites appear to have been 

 thus formed from limestones of organic origin by the action of 

 solutions containing magnesia. Other limestones have been con- 

 verted into iron-stones b}' the analogous reactions with solutions 

 containing iron, as in the Cleveland iron-stones. 



0. By gasolytic action, — as of vapors and gases, at times strong- 

 ly acid, as seen in solfataras and fumeroles, in decomposing and 

 reconstructing rocks ; thus are limestones often converted into gyp- 

 sum, kaolin, and clays derived from felspathic rocks, and manj'' 

 compounds of boracic acid formed. 



6. By a combination of two or more of the above agencies; in 

 some of the older deposits all appear to have combined, either at 

 once or at different times. Many mica schists appear to have 

 been originally micaceous sandstones, changed by long-continued 

 gentle heat and pressure, causing recrystallization ; so quartzites 

 are often recrystallized and indurated sandstones, or sandstones 

 cemented i)y gelatinous silica. 



Although he admits that hydrothermal action has taken part in 

 these metamorphic changes, he does not regard this, or the com- 

 bined effects of enormous pressure, heat, and water, as the grand 

 cause of metamorphism. He concludes that these changes in the 

 older strata have, in the main, been effected while these rocks 

 were in a perfectly solid condition, by what may be termed mole- 

 culo-chemical action, or a combination of chemical force and 

 molecular movement, which may have been brought into activity 

 by any one or more of the above agencies. 



TEMPERATURE OF THE EARTH's CRUST. 



However different may be the oiDinions of physicists as to the 

 condition of the internal nucleus of the globe, all are probably 

 agreed that there is an actual increase of temperature as we go 

 downward to an unknown depth. The occurrence of strata in an 

 unaltered state, supposed to have been buried beneath newer 

 strata to a depth of several thousand feet, is no argument against 

 this view ; for, at an average rate of increase of 1° F. for every 60 

 feet of depth, the boiling-point of water would not be reached 

 under 12.720 feet, a temperature under which it is doubtful if 

 metamorphism would take place in ordinary strata, and a depth 

 beyond which we rarely can fix the original position of rocks. 

 Experimental evidence, derived from the water of artesian wells, 

 coal-seams, and fissures in mines, and from observations made 



