HEMITHRENES AND OTHER CALCIT1C ROCKS. 47 



CHAPTER X. 



THE METHYLOTIC ORIGIN OF HEMITHRENES AND 

 OTHER RELATED GALCITIC ROCKS. 



IN Norway there are in several places, associated with ineta- 

 morphic rocks, crystalline calcareous masses, called by Scandi- 

 navian geologists "uhrkalk" or primary limestone. Besides 

 containing a large number of rare and interesting minerals 

 well developed, it is highly charged with ordinary mineral sili- 

 cates, often minute or microscopic, and distributed throughout the 

 calcareous matrix. Uhrkalk is thus a mixed metamorphic rock, 

 consisting of mineral silicates and mineral carbonates ; and in 

 this respect it answers to the hemithrenes and calciphyres, 

 respectively hornblendic ("amphibolique") and augitic (" pyroxe- 

 nique^), of Alexander Brongniart*". 



Having paid some attention to rocks of the kind, we are of 

 opinion that the distinction formulated by Brongniart cannot be 

 maintained for practical purposes ; we therefore prefer including 

 them in one group, retaining for it the name hemithrene, this 

 being the first introduced by the author in his ' Classification et 

 caracteres mineralogiques des Roches homogenes et heterogenes ' 

 (Paris, 1822) . 



We have already shown that calcite has supplanted minerals 

 which do not contain a trace of carbonate of lime. As pseudo- 

 morphs are developed from the change in question, it may be 

 legitimately inferred that if a certain rock-forming mineral 

 silicate, say augite, has the substance of its crystals replaced by 

 calcite, there can be no reason why a rock essentially composed 

 of the same mineral, and occurring under suitable conditions, 

 cannot become calcitized. Bischof, it would appear, was not 

 averse to this view ; for in speaking of granular limestone or 



* The name akerlyte was proposed for these rocks in my paper already 

 referred to ; but, from what is stated in the text, my colleague and I feel 

 it right to adopt Brongniart's earlier name in preference. W. K. 



