ENDLICH.J OEIGIN OF ERUPTED ROCKS. 267 



towards the centre of the earth. Thereby the isothermal zones within 

 the earth's crust will be changed. 



For the purpose of illustration, we will take the zone of liquefaction. 

 In case certain portions of the earth's crust are forced nearer towards 

 the centre of the earth, there will be a tendency towards re-establish- 

 ment of the disturbed equilibrium. Eocks and portions of rocks that 

 have heretofore been in a rigid condition have passed the zone of lique- 

 faction. Necessarily, this influx of heat-absorbing material will change 

 the absolute limits of this zone. A portion of the intruding material 

 will be liquefied ; another portion will assume a higher degree of tem- 

 perature ; a portion may or may not retain 'its former conditions. 



Primarily, additional pressure is exercised within this group by the 

 rocks changing from rigidity to plasticity, and to viscidity. According 

 to experiments by Bischoft', granite, for instance, loses more than 10 per 

 cent, of its volume in passing from a melted or plastic condition to rigid- 

 ity.* Eeversing the proposition, we find that a rock representing the 

 type that we might expect to find within the interior of the earth's crust 

 gains more than 10 per cent, by being melted. This pressure, again, 

 will exercise its influence in still further increasing the degree of heat, 

 and will thus produce a viscous condition of some portions that other- 

 wise would have remained plastic or perhaps even rigid. This influence 

 will make itself felt within certain limits, defined by the quantity of 

 originally remelted material. 



In this manner we may assume that what have been designated as 

 " reservoirs " are formed. In speaking of the condition of the earth's 

 interior, the term " liquid or plastic " has been used in previous pages. 

 Inasmuch as the questions involved did not directly affect this point, 

 the term was thus used. It seems to me that the hypothesis, supposing 

 the existence of a solid, anhydrous nucleus, and a liquid zone between 

 it and the rigid crust, is not sufficiently well sustained to demand accep- 

 tation. More in conformity with phenomena observed, and better 

 adapted for the support of many most important hypotheses and theo- 

 ries, is the assumption which regards the interior of the earth as being 

 in a plastic, anhydrous condition. Between thisanhydrous, central mass 

 and the exterior crust lies a thermal zone containing disconnected areas 

 of liquefied rocks. These are the reservoirs. Their extent is directly 

 proportionate to the amount of accumulated sedimentation, and to the 

 solvent powers of such agents as may be employed. 



As solvent agents we may regard primarily the directly acting pres- 

 sure, and, secondarily, the presence of water and other compounds. It 

 seems eminently probable, and, so far as experimental knowledge goes, 

 the supposition is supported, that water under great pressure and at high 

 temperatures will act as a most powerful agent in breaking up the com- 

 position of minerals constituting the rocks at such depths. In tnis con- 

 dition of hydrothermal solution we may assume that a hydration of some 

 of the compounds, so far as they may still be individualized, will take 

 place. Such hydration necessitates a decrease of specific gravity, an 

 increase of volume. It is impossible to measure the quantities to which 

 this would amount, because we are not able to reproduce, artificially,^ 

 the conaitions existing at the depths we are speaking of. Hunt says :t 

 ^'The mechanical pressure of great accumulations of sediment is to be 

 regarded as co-operating with heat to au2;mentthe solvent action of the 



* Bulletin de la Soc. Geol., 2d series, vol. iv, p. 1312. 

 t Am. Jour, Sci. and Arts, vol. iv, p. 26. 



