452 SKETCH TOWARDS A 



casual minerals, is the result of crystallization. Horn- 

 blende schist is the produce of fused clayslate ; being 

 compound or simple under different modes of cooling, 

 as, possibly also under some difference of materials ; 

 and though quartz rock has been exposed to heat, it 

 has not suffered fusion. Argillaceous schist has been 

 in the same state as micaceous schist, where it contains 

 crystallized minerals, and, possibly, always; since even 

 shells would not be destroyed in clay, though they are 

 in limestone. Primary limestones have been in fusion 

 wherever they contain crystallized minerals, and pro- 

 bably always ; with the obliteration of their shells. 

 Diallage rock appears to be analogous, both to granite 

 and gneiss : that is, to have been fused, either in situ, 

 or with effusion ; and this appears true also of serpentine. 

 And pitchstone is a trap, essentially; but the causes 

 of its peculiar (Character and limited place are not 

 quite apparent. 



It having been long admitted that the action of heat, 

 through igneous rocks, affects strata in contact, I have 

 shown that it also produces several concretionary struc- 

 tures ; among which, the columnar and the spheroidal 

 are most particularly demonstrated; as is the fibrous, 

 and also the schistose, if less common: and further, 

 that under any cause, the schistose structure of the 

 primary slates is concretionary. I have also proved 

 that such concretionary structures occur in rocks heated 

 short of fusion, and, moreover, that this passes gra- 

 dually into the crystalline ; proving a connection, rather 

 than an analogy, which adds to the difficulty of all our 

 theories on the subject of crystallization. If I have 

 further shown that flexures might have been produced 

 under the influence of water and fire both, so have I 

 proved, that the crystalline polarity extends through 

 large masses ; wh ence, as I have just said, the paral- 

 lelism of mica in the enrly rocks depends on this cause. 



