HEMITHRENES AND OTHER CALCITIC ROCKS. 59 



matter"*. Agreeing with Heddle that it was not the " sparsely 

 sprinkled" pseudomorphosed crystals "which have been the origin 

 of the lime stratum," we nevertheless ask why may not augitic 

 rook-masses, by methylosis and decretion into the ee trifling 

 specks," have given rise to it? Considering also that the marbles 

 of Harris and Lewis are Archaean, like those of Tiree, we 

 have no hesitation in placing them in the same category. 



Professor James D. Dana, as it seems to us., advocates the 

 methylotic origin of hernithrenes ; but he takes a different 

 view from ours as to the modus operandi of the process. In his 

 description of rocks of the kind common in Westchester 

 County, New York, he argues that they have been originally 

 limestones or dolomites, which, through the action of hot 

 silicic solutions, have become silicated, their mineral carbonates 

 (calcareous and magnesian) being thus converted into mala- 

 colite, tremolite, and other mineral silicates f. We have no 

 objections to urge against changes of this kind having occa- 

 sionally taken place. But in the rocks to which we refer the 

 evidences are so palpable and prevalent of the calcite having 

 replaced malacolite, that we feel assured, if Prof. Dana were 

 to decalcify specimens of the Westchester rocks, he would at 

 once see the force of our view in its application to his par- 

 ticular instances. Another objection lies against the idea that 

 hemithrenes are silicated limestones, inasmuch as, if applied to 

 the Archaean deposits of the kind, it would make the latter to 

 have been even more calcitic than they are at present ; and this 

 would increase the difficulty in solving the problem (to be dis- 

 cussed in a subsequent Chapter) as to the source which supplied 

 the calcite. 



* The Tiree marble evidently contains much more augitic residue than is 

 represented by the crystalloids, as there are frequently imbedded in it siliceous 

 bodies of various sizes : the crystalloids are also often surrounded by a white, 

 spongy, siliceous covering (corresponding to flocculite), to be seen after 

 decalcification. Macculloch has noticed something of the kind (op. cit, 

 vol. i. p. 53). 



t American Journal of Science, 3rd ser. vol. xx. p. 28, &c. The origin of 

 these rocks will be further noticed in Chapter XIII. 



