84 ROCK-METAMORPHISM. 



It is not until the Lower Silurians are reached that limestones 

 are found to occur to any extent ; and these are either very 

 impure, as in the Llandeilo flags, or they form thin beds, e. g. 

 those in the Bala limestone of the Caradocs. The Durnes lime- 

 stone in Sutherlandshire (a partly mineralized rock, older than 

 our Llandeilo flags, and probably the equivalent of the Stiper- 

 stones, the bottom of the Lower Silurians) and the Coniston 

 limestones may be regarded as improvements on their Welsb 

 equivalents. 



During the Upper Silurian period a marked change took 

 place : limestones were developed on the grandest scale ; and 

 since then the formation of the same class of rocks to the like 

 extent has continued throughout every succeeding period. 



On the continent of Europe a similar scarcity of limestones 

 characterizes the Cambrians, while a fair increase in their amount 

 marks the Lower Silurians. 



In North America the Cambrian limestones offer no very 

 marked exception to contemporaneous formations in the British 

 Isles or on the Continent. 



Of the Acadian and Potsdam groups, which have been 

 bracketed with the Welsh Llongmynds, Harlech Grits, and 

 some other Lower Cambrians, the first contains no recorded non- 

 crystalline limestones*; while the second only comprises some 

 of inconsiderable thickness and occupying but small areas : such 

 are the red sandy dolomites and other calciferous formations of 

 Troy (N. B.), North-western Vermont, the Straits of Belle Isle 

 (Newfoundland) , and a few more places. 



and covered with flocculite (which substance can only be their residue) than 

 where they are sharp and translucent. The crystalloids of malacolite in a 

 few cases were observed to be so far decreted as to assume rude branching 

 forms. In one instance of this kind the form closely resembled typical imi- 

 tative configurations. Intermixed with the calcite and malacolite we found a 

 pale greenish-yellow granular substance resembling serpentine, bundles of 

 prismatic epidote or actinolite, magnetite in octahedral crystals, a triclinic 

 feldspar showing striping, and galena. This case strikingly resembles the one 

 at Cleggan. If these specimens are characteristic of the limestone beds at 

 Porthlisky, there can be no question about the latter being methylosed 

 products. 



* For reasons which will be understood after a perusal of the last para- 

 graph of the present Chapter, we confine our remarks on the question under 

 consideration to unaltered limestones. 



