AND ANALOGIES, OF ROCKS. "207 



action of heat ceases. Yet I must remark, that the 

 action of heat on the argillaceous schists may have 

 been short of producing this effect, and yet have 

 caused others analogous to those which occur in 

 micaceous schist. Such is the formation, within them, 

 of hollow spar or other minerals, which we cannot 

 attribute to any cause but this. 



Thus, two important members of the primary strata 

 may have been indurated from water alone ; and ye* 

 they appear also to have, in some cases, been affected 

 by heat, perhaps even indurated by it. With respect 

 to limestone, it is now known, both by direct experi- 

 ment and by observation on the effect produced by 

 trap veins on chalk, that it may be crystallized from 

 fusion, provided the escape of the carbonic acid is 

 restrained. It has been shown that it is equally 

 consolidated from water ; and on examining this lime- 

 stone in its various associations, its origin must pro- 

 bably, in some instances, be referred to one of these 

 causes, in others, to the other. It is probable, for 

 example, that all the limestones associated with clay 

 slate are derived from watery deposition and crystal- 

 lization ; though even these may, like the slate itself, 

 have. been affected by heat; but it is much more than 

 probable that those associated with gneiss have received 

 their present condition from the latter agent. This 

 opinion is justified by many circumstances ; such as by 

 their giving passage to granite veins, by the change of 

 chemical texture and composition which they present 

 in these cases, and by the crystallization, within them, 

 of minerals similar to those found in gneiss, such as 

 garnet, hornblende, augit, and others, which could not 

 have been deposited from water so as to have entered 

 into the confused crystalline arrangement of the rock 



