208 Effects of Heal modified hy Compression. 



arrangement*; and the substance, which, in this sluggish 

 state, would be little disposed to move, being confined in 

 its original situation by contiguous beds of more refractory 

 matter, would crystallize without undergoing any change of 

 place, and constitute one of those beds of whinstone which 

 frequently occur interstratified with sandstone and lime- 

 stone. 



In other cases, where the heat was more intense, the 

 beds of sand, approaching more nearly to a state of fusion, 

 would acquire such tenacity and toughness, as to allow 

 themselves to be bent and contorted, without laceration or 

 fracture, by the influence of local motions, and might as- 

 sume the shape and character of primary schistus ; the lime- 

 stone would be highly crystallized, and would become mar- 

 ble, or, entering into thin fusion, would penetrate the mi- 

 nutest rents in the form of calcareous spar. Lastly, when 

 the heat was higher still, the sand itself would be entirely 

 melted, and might be converted, by the subsequent effects 

 of slow cooling, into granite, sienite, &c; in some cases, 

 retaining traces of its original stratification, and constituting 

 gneiss and stratified granite ; in others, flowing into the cre- 

 vices, and forming veins of perfect granite. 



In consequence of the action of heat upon so great a quan- 

 tity of matter, thus brought into a fluid or semifluid state, 

 and in which, notwithstanding the great pressure, some sub- 

 stances would be volatilized, a powerful heaving of the su- 

 perincumbent mass must have taken place, which, by re- 



* This state of viscidity, with its numberless modifications, is deserving of 

 grea"t attention, since it affords a solution of some of the most important geo- 

 logical questions. The mechanical power exerted by some substances in the 

 act of assuming a crystalline form, is well known. I have seen a set of large 

 and broad crystals of ice, like the blade of a knife, formed in a mass of clay 

 of such sthTuess that it had just been used to make cups for chemical purposes. 

 In many of my former experiments I found that a fragment of glass, made 

 from whinstone or lava, when placed in a muffle heated to the melting point 

 of silver, assumed a crystalline arrangement, and underwent a complete 

 change of character. During this change it became soft, so as to yield to the 

 touch of an iron roc}; yet retained such stiffness, that, lying untouched in the 

 muffle, it preserved its shape entirely, the sharp angics of its fracture not 

 being in the least blunted. 



peated 



