Physical and Chemical Behavior of Solids. 217 



considered his pressure uniform, and gives an explanation on 

 that basis, essentially similar to that above. From his mode of 

 experiment it is obvious that his pressures may have been so 

 nearly uniform that the inequality of the pressure on solid and 

 liquid was not sufficient to effect an absolute increase in the 

 solubility of the above two substances. Under geological con- 

 ditions it is probable that such a case is exceptional, firstly 

 because there are relatively few substances belonging to the 

 second category above, and secondly because even with these 

 substances it can occur only under specified, and to some 

 extent exceptional, conditions of compression. 



Another piece of evidence may be cited from Spring, who 

 writes* as follows: We may suppose "that the component 

 ,sand grains of sandstone, or the rolled pebbles of pudding- 

 stones, have been in contact with a supersaturated solution of 

 silica produced by the agency of pressure, and that this solu- 

 tion, in unstable equilibrium, [that is, wherever the pressure is 

 released] has furnished the cement necessary for consolidation. 

 It is not without interest to determine if there really exists 

 between the component granules a silicious material which 

 escapes direct observation. Now dry amorphous silica has the 

 property of dissolving slowly in a solution of alkali hydroxide, 

 whereas quartz grains are refractory or are attacked by alkali 

 only with extreme slowness. A block of sandstone or of pud- 

 ding-stone ought then to be disintegrated in a solution of 

 potash, if there is really a layer of silica, however fine, between 

 the grains. Experiment has confirmed this deduction : all the 

 quartzose or schistose rocks which have been examined have 

 disaggregated in an alkaline liquid, but with greater or less 

 rapidity according to their nature. Sandstones of relatively 

 recent formation, such as tertiary sandstones (landeniens) or 

 secondary sandstones (keupriques), have required only some 

 weeks at 100° to be disaggregated, while older sandstones, 

 psammites and especially pudding-stones, have been much more 

 resistant to alkalies. The latter become only more or less fri- 

 able, whereas the former were changed into mobile sand. 

 The reason for this greater resistance is obvious. In all proba- 

 bility it consists in the circumstance that the silica which 

 cemented the grains together has in the lapse of time been 

 transformed into quartz, which is refractory towards alkali." 



This view is confirmed by a few experiments made by 

 Spring, which showed that sand grains may be cemented 

 together by keeping them under slight pressure in contact with 

 colloidal silicic acid, while the latter slowly dried. 



*Bull. Acad. Eoy. Belg. (3), xxxvii, 810-12, 1899. 



