20 Trof W. King — On the Structure of a Rochfrom Ceylon. 



depending on the nature of their respective acid constituent. The 

 first comprises syenites, gneisses, and ordinary crystalline schists — 

 all essentially composed of mineral silicates. The second includes 

 ophites, urkalks,* and some others, which, in addition to siliceous 

 minerals, contain more or less mineral carbonates.^ 



It is chiefly with the second group that the present communication 

 is concerned. 



Although a number of highly valuable researches on the micro- 

 scopic structure of the silaoid rocks have been published, it is 

 remarkable that very little attention seems to have been paid to 

 similar investigations in the silo-carbacid group. Excepting certain 

 papers that have appeared respecting one of its members, noted for 

 containing the so-called " Eozoon " (the constructors of which, 

 however, took little notice of anything that was clearly of crystal- 

 line origin), geological literature presents a blank as regards the 

 microscopy of the group • the more surprising, as, apart from the 

 belief in the above reputed "organism," the silo-carbacid rocks form 

 a subject of considerable importance in theoretical geology ; the 

 question as to their origin having evoked at different times some 

 lively discussions, and the most opposite views. I cannot, therefore, 

 too strongly recommend them for examination to investigators of 

 micro-lithology. 



Fortunately, the examination of the silo-carbacid rocks is com- 

 paratively inexpensive, and attended with imcommon facilities. No 

 sections, unless for supplementary, or optical purposes, are required. 

 Mere chips, after remaining in a weak solution of hydro-chloric acid 

 for a few hours (decalcification, as it is called), will display under 

 the microscope the siliceous minerals freed from the associated 

 mineral carbonates, with every peculiarity of form, condition, and 

 grouping, in a highly instructive manner. 



In conjunction with my colleague, Dr. T. H. Kowney, I have lost 

 no opportunity of late years in investigating the structure of these 

 rocks ; while in the joint memoirs we have published many of our 

 observations are minutely described.^ 



serpentinous argillites, etc.---are intimately associated with unaltered rocks. A 

 deposit in Co. Antrim, agreeing with typical beauxite in being a hydro-oxyferriferous 

 alumina (and which has been worked by Dr. Ritchie, of Belfast, who has kindly 

 furnished me with specimens), is seemingly a methylotic product, derived from basalt 

 by the elimination of silica, and the addition of water, etc. 



' I adopt this name from Werner, tlioui;h it is restoring one about as objectionable 

 as gneiss and some others currem- in Geology. I have long thought- of proposing the 

 name akerlyte for a similar crystalline calcareo-siliceous rock, occurring at Aker m 

 Sweden ; but probably one of Brongniart's names, calciphyre and hemitrene, of a 

 later date than Werner's, will prove to be a synonym. 



2 Apatite and magnetite occasionally prevail to such an extent amongst the 

 Laurentian metamorphics of Canada ns to form phosphatio and oxidic sections of the 

 silo-carbacid group. There are other rocks present, in which mineral carbonates 

 occur so sparsely as to merge them closely into thf silacid group; on the other hand, 

 it is conceivable that others exist in which the mineral silicates may be reduced to a 

 very small per-centage. 



3 See Quarterly Journal of the Geological Society, vol. xxii., 1866 ; and vol. xxv., 

 1869; also Proceedings of the Koyal Irish Academy, vol. x., 1870; and Id., new 

 series, vol. i., 1871. 



