Lecture oti the Chemistry of Geology. 247 



ly, to the productioD, by means of aqueous solution, of sili- 

 ceous and other deposites, which were commonly regarded as in- 

 soluble. He would touch cursorily on the former, chiefly with 

 a view to.|ajal^f^t<^^^^<;^^gr^|}eDsion of the latter. 



y^\jr^ Disintegration of Rocks. — The principal agents concerned 

 in the disintegration of rocks, might, it was said, be conveniently 

 arranged, under three heads : — 



^R^ Mechanical agents ; such as rain, rivers, and torrent8> or, 

 generally, water in motion. This subject, the lecturer said, did 

 not require comment on that occasion, as it was not only fami- 

 liar to the geologists, but foreign to the plan of his lecture. ^ 



2. The alternate congelation and liquefaction of water. In 

 all situations liable to alternate frost and thaw, this was a most 

 fertile source of destruction to rocks. 



Water, insinuating itself into fissures, or between the strata 

 of rocks, and congealing there, tore asunder the firmest masses 

 by the immensely expansive force which water exerts in freez- 

 ing, kept together the disjointed parts as by cement while it 

 remained solid, and, on thawing, left them to fall asunder by 

 the mere force of gravity. This was perhaps the most influen- 

 tial cause of the vast ruin daily witnessed in the valleys of Swit- 

 zerland, and in all countries where high mountain-chains are inter- 

 sected by deep narrow gorges, bounded by bare precipitous and 

 irregularly-fissured escarpments. By the operation of the same 

 cause, buildings were defaced and destroyed. When water 

 froze within the cavities of porous stones, the particles were fre- 

 quently more or less disunited from each other, and crumbled 

 into dust at the first thaw. Building materials differed in their 

 destructibility by frost. The compact tenacious sandstdrte of 

 Edinburgh suffered little, while some of the handsome Colleges 

 of Oxford gave melancholy proof '6f*i1i6 iiijufy^ Mii6h it nVight 

 occasion in the more porous and less tenacious oolite of that 

 country. The lecturer observed, that a scientific knowledge of 

 the cause of such decay had led to the suggestion of a ready 

 mode of estimating the durability, as far as froSt was concerned, 

 of different building materials. The freezing of water was a 

 process of crystallization attended, as in most other cases, with 



