HARITY OF POST-ARCHAEAN LIMESTONES. 83 



CHAPTER XIII. 



WHY ARE LIMESTONES COMPARATIVELY RARE IN 



THE FORMATIONS IMMEDIATELY SUCCEEDING 



THE ARCH^EANS ? 



IT has long been a matter of surprise that the Cambrians 

 (the great series of rocks from the base of the Longmynds to 

 the top of the Tremadocs), exceeding by far 40,000 feet in 

 thickness, contain very few limestones; and the surprise is 

 heightened when the paucity of the latter formations is compared 

 with the vast masses of calcareous members belonging to the 

 Archseans. We are referring to the Cambrians as they occur in 

 Wales, Scotland, and Ireland, where their calcareous matter, 

 comparatively a mere fractional constituent, is generally in a 

 diffused state, or forming only thin layers. 



Even by including Hicks's Dimetian series (which, however, 

 he regards as Archaean) , the ' ' limestone beds " it contains at 

 Porthlisky would form no valid exception to our statement, 

 particularly as there are strong grounds for the opinion that 

 they, and the ophite associated with them, do not retain their 

 original composition*. 



* Dr. Hicks has kindly favoured us with two small specimens taken from 

 the " impure limestone bands " (from 1 to 3 feet in thickness) of Porth- 

 lisky, and belonging to his Dimetian series. This is separated by uncon* 

 formity from his Pebidian series. He is inclined to consider both as Archaean, 

 the Pebidians being overlain unconformably by unaltered Harlech grits. We 

 find the specimens to be principally composed of white augite or malacolite, 

 in short well-cleaved crystalloids confusedly aggregated. After decalcifi- 

 cation they present themselves separated by interstices, cavities, and con- 

 tinuous passages, which before had been filled with calcite. In many 

 instances the crystalloids of malacolite are translucent, and their angles are 

 sharp ; but often they are opaque, rounded, and incrusted with white floccu- 

 lent matter. Eveiy thing observed in connexion with the malacolite and 

 calcite convinces us that the former is in course of replacement by the latter. 

 The calcite is more abundant where the crystalloids of malacolite are eroded 



