MINERALIZED CRYSTALLINE LIMESTONES. 89 



CHAPTER XIV. 



SOME CRYSTALLINE LIMESTONES AEE SIMPLY 

 MINERALIZED. 



IT will be recollected that we have made an exception to cer- 

 tain crystallized calcareous rocks related to hemithrenes being 

 of purely methylotic origin. It seems probable that this excep- 

 tion may apply to the marbles of Dalnein in Strathdon, Glen 

 Elg, Glen Tilt, and the neighbouring localities : still we cannot 

 altogether reject the idea that there are nomethylosed examples 

 amongst these marbles, even should they be of post- Archaean age, 

 just as it may be doubted that all the Westchester crystalline 

 limestones are simply mineralized products. The fact men- 

 tioned by Dana that " green hornblende in minute rounded 

 crystals is occasionally found disseminated through the lime- 

 stone of Westchester/'' and the similar well-known fact of the 

 Scotch marbles being shotted with decreted crystalloids of augite 

 &c., involve the action of corroding or dissolving solutions the 

 prime agents in methylosis which marbles, according toHeddle, 

 contain trifling specks or remains of augite, and are of post- 

 Archaean age ; and for this reason doubts may be entertained of 

 their belonging to the group treated of in Chapter X. 



But we see no improbability of an impure limestone belonging 

 to the Cambrian, Silurian, or later systems *, through mineraliza- 

 tion, changing into a rock which it would be difficult to distin- 

 guish from a hemithrene f. Therefore, until further light has 

 been gained respecting the geological position of the marbles of 

 Glen Elg and other places referred to by Heddle, and generally 

 considered to be Upper Cambrian or Lower Silurian in age, we 

 consider it safest to admit that some of them may have been 

 calcareous in their original condition, especially as the Durness 

 limestones are proved to be early Palaeozoic by their fossils. 



* Some of the Scandinavian " primary limestones," described by B. Gotta 

 (Zeitsclirift der deutsch. geol. Gesellschaft, 1852, Band iv. pp. 47-53), are 

 undoubtedly of this class ; but we strongly suspect that many others he has 

 noticed (Aker &c.) are methylotic. 



t Excluding its ophitic portions, much of the Isle-of-Skye " white marble " 

 may be an example of mineralized limestone, 



