24 Prof. W. King — On the Structure of a Rock from Ceylon. 



disconnected blocks and lenticular sheets, — peculiarities held to be 

 inexplicable on the idea that they are in any way related to 

 sedimentation. "While some have left the difficulty to be cleared up 

 by future investigations, a few who have studied pseudomorphism 

 in minerals meet it by contending that in the cases referred to the 

 same phenomenon, accompanied by new mineral arrangements, has 

 been developed. 



So much may be urged in favour of this view, that I, with my 

 colleague, have decided on adopting it to explain, not only the origin 

 of urkalks and ophites, including the rocks last noticed ; but also 

 the " great beds of limestone," which in Canada characterize the 

 Laurentian system. The same laws by which felspar, garnet, and 

 other mineral silicates are changed into calcite (cases of the kind 

 being well known), can operate under the necessary conditions — 

 say, hydro -plvitonic — in converting widespread strata of silacid rocks 

 into the above so-named beds. Most of the latter, there are strong 

 grounds for believing, are urkalks : some of them, however, may 

 be so sparsely charged with mineral silicates as to justify to some 

 extent, and in their particular case, the name that has been given 

 to the entire series by the Canadian geological surveyors. 



In considering the order of the changes which the Laurentians, as 

 a whole, have passed through, according to the doctrine under dis- 

 cussion, it must not be overlooked that this great Eock-system 

 includes, besides its vast beds of every variety of syenite, gneiss, 

 etc., other members which, although essentially formed of the 

 minerals constituting such beds, contain chrysolite, loganite, serpen- 

 tine, malacolite, chondrodite, pyrallolite, talc, pargasite, magnetite, 

 woUastonite, apatite, etc.,' with appreciable amounts of mineral 

 carbonates : "^ the members alluded to may be referred to the first 

 stage. Typical ophites and urkalks, still largely made iip of the 

 above mineral silicates, but in which there is a notable increase of 

 dolomite, or calcite, ought to be considered as belonging to the 

 second stage. While the purer "limestones," in which it may be 

 assumed the mineral carbonates have gained the ascendency, will 

 have to be allocated in the third stage. 



In conclusion, taking for granted that the Laurentian rocks are 

 metamorphosed sediments (silicates) derived from earlier or pre- 

 existing masses of similar chemical composition, I have no hesita- 

 tion in accepting the conclusion — that the methylosis herein roughly 

 sketched out, or some analogous process, began after the sediments, 

 affected by it, had become mineralized into syenites, gneisses, and 

 the like ;^ and ended by their being converted into " limestones." 



^ "WTiatever particular view may be taken of pseudomorphic processes, I am 

 strongly inclined to believe that the various minerals mentioned in the text are 

 secondary products, resulting from chemical changes in syenites, gneisses, etc. 



2 Kalk-diorite and kalk-glimmerschiefer of the Germans seem to belong to this 

 group. Kalk-diabase and Iherzolite appear to be younger rocks, whose calcite may 

 be original, or it may have been derived from limestones older than themselves ; and 

 I am disposed to admit that much of the calcitic urkalk (not the dolomitic) of Con- 

 nemara, apparently Lower Cambrian in age, may be a simply metamorphosed 

 calcareous rock. 



3 It is even probable that chemical changes commenced while the Laurentian 



