58 ROCK-METAMORPHISM. 



after being decalcified, showed separated grains or crystalloids of 

 malacolite with a thin white crust enclosing their translucent sub- 

 stance ; others were seen with a portion of the latter removed, 

 but the crust remaining intact ; while close by were hollow 

 spheroidal cases identical in composition with the crusts: before 

 decalcification the cases were filled with calcite. A few examples 

 of the cases occur entirely riddled, reminding one of the per- 

 forated shell of a Polycystine *. 



In our remarks on these things it was contended, considering 

 the crystalloids of malacolite show themselves in every stage of 

 decretion, that in numerous instances they must have disappeared 

 altogether ; also that it was equally to be inferred that the inter- 

 stitial calcite separating the grains, even that forming the asso- 

 ciated layers, had replaced a corresponding amount of malacolite. 



From the vai'ious evidences just given, added to those pre- 

 viously adduced, all demonstrating that both minerals and rocks 

 essentially silicates have been converted into calcite and cal- 

 careous masses, we feel it impossible to form any other con- 

 clusion, as regards the hemithrenes so far considered, than that 

 they were originally silacicl metamorphics, also that the asso- 

 ciated marbles, <( consisting of nearly pure carbonate of lime/' 

 are examples in which the mineral silicates have been completely 

 extracted by the same methylotic process. 



Professor Heddle, it may be said, takes a different view. 

 Referring to the hemithrenes that have come under his own 

 observation, such as occur in Tiree, Harris, Lewis, and some 

 other localities in Scotland, and contending that the crystalloids 

 they contain " are one and all pseudomorphs of preexisting 

 crystals of augite, sparsely sprinkled throughout a great mass of 

 lime, in an amount which is altogether quite trifling," he 

 declares that " these trifling specks could never have been the 

 origin of a lime stratum tens of feet in thickness "f. 



Whatever may have been the origin of the other hemithrenes 

 referred to by Heddle, we certainly must contend, with respect 

 to the Tiree marbles, that their rounded crystalloids are not 

 only pseudomorphs but also the residue of what was once an 

 augitic rock, and that the portion which has been destroyed, or 

 rather removed, has undergone replacement by calcareous 



* Proc. Koy, Irish Acad, n. s. vol. i. p. 138 (1871;. f Op. cit. p, 542, 



