48 ROCK-METAMORPHISM. 



saccharoid marble when it occurs as <f subordinate beds in 

 crystalline slates;" he considers that in this case " the carbonate 

 of lime may have originated from the decomposition of cal- 

 careous silicates "*, which, it must be understood, are frequently 

 present in the latter rocks. 



That such cases have already been made known by Scheerer, 

 Delesse, and others, though their origin has not been rightly 

 understood, seems highly probable. 



Leonard* Horner, in his Presidential Address to the Geological 

 Society, 1861, remarked that "many instances have been met 

 with of granular limestone occurring under circumstances that 

 can only be explained by supposing them to have had a sub- 

 terranean origin." Besides other instances, which Horner 

 might have had in his recollection, probably the following, cited 

 from MacCulloch, was not unknown to him : it consists of an 

 " irregular or nodular mass of limestone (pink coccolite marble) 

 enclosed in gneiss without any connexion or continuity "t- 



Recently, Professor Heddle, referring to the " primary lime- 

 stones" of Shetland, has stated that they are "often very 

 obscurely or imperfectly stratified, while occasionally they show no 

 marks of that deposition, but rather seem to form, like serpentine, 

 large imbedded shapeless masses, or huge irregular nodules "J. 



Selwyn, speaking of the dolomites and limestones associated 

 with the " diorites, dolerites, amygdaloids, and volcanic agglo- 

 merates," presumed to belong to the Levis division of the 

 Quebec rocks in Canada, has lately remarked that they ' ' have 

 much more the appearance of great lenticular, vein-like, cal- 

 careous masses than of beds belonging to the stratification ". 



Putting aside a number of cases which could be adduced from 

 different writers, we shall confine ourselves to a few with which 

 our personal observations have made us intimately acquainted. 



In the summer of 1877, F. Twining, Esq., of Cleggan Tower, 

 north of Clifden, Connemara, drew our attention to a dyke 

 (not marked on the map of the district by the Irish Geological 

 Survey, which had just been published) on the north shore of 

 Cleggan Bay, immediately adjacent to a small rocky islet or, 



* Chem. & Phys. Geology, vol. iii. p. 140. 



t Western Islands of Scotland, vol. i. p. 48 (1819). 



J The Mineralogical Magazine, vol. ii. p. 118 (September 1878). 



Canadian Naturalist, vol. ix. p. 24 (February 24, 1879). 



