CHEMICAL AND DYNAMICAL GEOLOGY. 81 



tossing sea, necessary to grind to powder such vast quantities of shells, without 

 scattering the bones of the fishes, birds and reptiles more than has been done, 

 without leaving indications of wear on the ammonite shells, or without even 

 shaking from its bed that piece of putrefying saurian skin. 



In the early days of geological science, almost every one believed that all 

 crystalline rocks and minerals were of igneous origin. In the course of time 

 another school arose, teaching that all crystalline rocks were of sedimentary 

 origin. These are known respectively as the plutonian and neptunian schools. 

 Modern investigations have brought so much support to the neptunists that the 

 leading geologists of to-day hold with them that crystalline rocks are of sedimentary 

 origin. Many deposits originally contained all the elements found in crystaUine 

 rocks; many others, long after they were formed, may have had new elements 

 added by infiltrating waters. The degree of porosity, as before shown, is sufficient 

 to permit such to be done. Internal heat is also an important factor in metamor- 

 phic changes. The average rise of temperature as we descend below the surface 

 is one degree F. for about sixty feet. In earlier times, when the outside crust 

 was comparatively thin, this increase was much greater. As deposits increased 

 in thickness the isothermal horizons would rise so that the time would come when 

 they would be heated to a high temperature. The conditions would then be 

 favorable for numerous changes in the sediments themselves. Waters containing 

 alkaline carbonates under heat and pressure will soften almost any silicate 

 Daubree has shown that the temperature need not exceed 700° F. for quartz 

 feldspar and pyroxine to be softened sufficiently to assume the crystalline form 

 upon cooling, and also that at this temperature alkaline silicates in solution will 

 unite with clay to form feldspar and mica. He also showed that the alkali 

 springs of Plombieres, with a temperature of 160° F., had acted on the bricks 

 and cement of the old Roman baths so as to form zeohtes and other silicated 

 minerals. Different geologists, notably, Mr. Sorby, from their study of crystals, 

 have concluded that granitic and trachytic rocks have been formed in the presence 

 of a liquid under great pressure and at a temperature not exceedino- that of 

 redness. These semi-fluid masses, under such, great pressure as they probably 

 endured, might frequently be forced into crevices and faults in the rocks above 

 them, and thus present the appearance of igenous rocks. 



The question of the origin of volcanoes is one intimately connected with 

 metamorphic changes. The old idea that the lava comes from the molten in- 

 terior of the earth has many facts in its favor. The greatest of these is the vast 

 extent of simuhaneous volcanic action in former times. According to Dana at 

 the close of the Triassico-Jurassic period, an eruption was continuous from Nova 

 Scotia to South Carohna ; the lava throughout the whole extent being doloritic. 

 Much more extensive eruptions have taken place on the Pacific slope. If I 

 rightly understand Dana he thinks that these vast eruptions originated in the 

 molten interior. However, he says, that if such be true, it does not follow that 

 all lavas come from the same source. 



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