Crystalline Limestone. 365 



at once admit, that the change has been partly or wholly pro- 

 duced by this contact. But how did this take place \ The 

 knowledge, that burning liquid masses sometimes burst forth 

 from the interior of the earth ; and, in addition, HalPs ex- 

 periments, naturally led to the idea, that these contact rocks 

 were at one time in a melted condition, and thus produced 

 the conversion by means of heat. Besides, indeed, we find, 

 that the rocks in question are, in most cases, those respecting 

 which it is, at least at present, assumed, that they were at 

 one time in a melted condition ; nay, we perhaps find even 

 examples of the masses in contact being undoubtedly pyro- 

 genic, and having thus been able to act upon the limestone 

 when in a hot state. When, however, we pursue our examina- 

 tion still farther ; when, from the love of truth, we do not fear 

 to encounter facts, which may, perhaps, overturn the result thus 

 obtained, it may really happen, in consequence of what obser- 

 vation has shewn, that we at last meet with such facts, and 

 that we are actually placed in the position of seeing our first 

 conclusion overthrown. Thus, it is not without example, that 

 the conversion of uncrystalline limestone into marble is met 

 with near masses, that either have never possessed the high 

 temperature that is usually presumed to have existed in such 

 instances, or, at least, not since they were in contact with the 

 limestone. Such is the case with a mass of the well-known 

 bone-breccia observed by H. Bronn, on the coast of the Me- 

 diterranean ; and F. Hoffman saw, in Sicily, limestone repos- 

 ing on basaltic tuff^ which was altered in the same manner as 

 happens when it is in contact with solid basalt. 



The cautious and reflecting observer will, however, not re- 

 quire facts of this description, in order to find it necessary at 

 this point in the investigation, to defer deciding, in the mean 

 time, in favom* of the explanation of the contact-marble by 

 means of heat. He will feel the importance of the circum- 

 stance, that the phenomenon of conversion, in the greater num- 

 ber of instances, occurs in proximity to rocks, whose former 

 melted condition is not undoubted, but is only for the moment 

 assumed. He will become so much the more circumspect at 

 this stage, when he discovers that the theorists, from whom 

 this opinion emanated, have, in fact, founded it chiefly on the 



