I'rof. W. King — On the Structure of a Rock from Ceylon. 23 



are now strongly inclined to regard it as wollastonite : be this as it 

 may, the beautiful configurations it has given rise to, and which 

 have been elsewhere noticed by us,^ are strictly identical in every 

 thing with the previously described examples. The Aker urkalk 

 does not appear to be so gemmiferous as the one from Ceylon, it 

 having only yielded to us violet-coloured spinels and fragments of a 

 green vitreous mineral that looks like chrysolite. 



The late Dr. Davy, in his memoir, " On the Geology and Miner- 

 alogy of Ceylon," written in 1818, and published in vol. v. of the 

 Transactions of the Geological Society, notices a " calcareous spar 

 variety of Gneiss," and a "dolomitic rock": both are probably 

 similar to the urkalk described in this communication. It would 

 appear from his account that the silacid rocks form the matrix of the 

 numerous precious stones found in the alluviums of Ceylon. I 

 admit that carbuncles, topazes, chrysoberyls, etc., may have been 

 derived from the gneisses, and the usually associated crystalline 

 schists ; but from what the urkalk reveals under the microscope I 

 am led to believe that calcitic or dolomitic rocks of the kind have 

 yielded the more valuable gems. I cannot speak with confidence 

 that sapphire is present in any of Captain Bayley's specimens ; but 

 as it is found in the urkalks of the United States, Ural, Hindostan, 

 China, and other countries, the fact may be accepted as strongly 

 confirmatory of my belief. 



It has long been known that the silo-carbacid rocks contain a 

 variety of minerals with the angles and faces of their crystals 

 frequently in a rounded condition. Some have suggested that this 

 peculiarity is the result of partial fusion ; while others contend that 

 it is the effect of the solvent action of heated watery solutions. 

 The latter is the view which has been adopted by myself and 

 colleague. I am not aware, however, that any one, except ourselves, 

 has offered the solvent theory to account for the present form of the 

 vai'ious configurations in serpentine, malacolite, and wollastonite, 

 characterizing ophites and urkalks ; or that the process it involves 

 eventually accomplishes their total removal. Careful observations 

 show, as it were, the process in operation in these rocks, — com- 

 mencing with the disintegration of their component mineral silicate, 

 — next, gradually removing it (isolated portions of which, through 

 irregular decretion, are made to assume all the imitative shapes 

 characteristic of the configurations), with simultaneous replacement 

 of dolomite, or calcite, — and ceasing on its complete elimination. 

 The phenomenon is evidently due to chemical changes or methylosis, 

 consequent on the presence in the rock of a permeating solvent 

 containing carbonic acid (? atmospheric in its origin), simple, or in 

 combination — an agent fully capable of displacing the silica of the 

 configurations, and substituting a new constituent. 



Furthermore, many geologists have been struck with the peculiar 

 modes of occurrence of silo-carbacid rocks, — intrusive or eruptive- 

 like, confusedly crumpled between comparatively undisturbed beds, 



* Proceedings of the Royal Irish Academy, vol. x., 1870. 



