Prof. W. King^^On the Structure of a Rock from Ceylon. 21 



Opliites and urkalks respectively represent two sections — liydrous 

 and anhydrous — into which the silo-carbacids may be divided. The 

 first consists essentially of the hydrated siliceous mineral, serpentine 

 and its varieties, more or less intermixed with mineral carbonates, 

 chiefly calcite and dolomite. The second is constituted of anhydrous 

 siliceous minerals, related to amphibole and pyroxene, associated 

 with mineral carbonates. Various other species — silicates and 

 carbonates, and some with different acids- — are rarely absent in both 

 sections. 



The presence of serpentines, or of amphiboles, is readily indicated ; 

 the former by having usually a conchoidal, and the latter an even-- 

 plane (cleavage) divisional structure. Both kinds of minerals also 

 occur in the granular, asbestiform, lamellar, and other conditions. 



Silo-carbacid rocks seem to be seldom absent wherever meta- 

 morphism has played. an important part; and, although undoubtedly 

 of widely different geological ages, they are found most extensively 

 developed in the very oldest systems, especially the Laurentian. It 

 was on this account that Werner named the rocks of the anhydrous 

 section urkalks. 



During the present year several varieties of ophite and urkalk 

 from India and Ceylon have come under my notice. They are of 

 such interest in many respects that I hope to be able, along with my 

 colleague, to describe them in detail on a future occasion. I have 

 thought it necessary, however, to draw up the present preliminary 

 paper, as some time may elapse before our observations on all these 

 acquisitions are completed. 



Captain Bayley, P. and 0. agent at Galle, has kindly furnished 

 me with the specimens from Ceylon, as well as a number of precious 

 stones. The former were procured on an estate of his situated 

 adjacent to and below the Haughton Plains. I am not in a position 

 to speak of the mode of occurrence or geological age of the rock, 

 which for some reason or other is called dolomite ; but being evi^ 

 dently a member of the great metamorphic series, so remarkably 

 developed in the island, it may very probably be Primordial. 



As is the case with many of the so-called dolomites occurring in 

 Canada and elsewhere, the rock under notice contains but an insig- 

 nificant amount of any mineral carbonates ; silicates being the 

 predominant constituents. 



By slightly decalcifying portions of it, the carbonates, principally 

 calcite, are partially dissolved out, and the silicates remain. Under 

 the microscope the latter are seen to consist for the most part of 

 what ajDpears to be wollastonite (silicate of lime), in irregular-sized 

 grains confusedly aggregated, or occurring singly, — each grain being 

 distinguished by its individual set of cleavage divisions. At first 

 sight this mineral might be taken for malacolite (silicate of lime 

 and magnesia) — a variety of pyroxene ; but careful observations 

 show that it has two prismatic cleavage intersections, which form 

 a more compressed rhombic solid than the one belonging to the last- 

 named mineral ; the prism, in short, more closely resembles the 

 primary of amphibole. Dr. Rowney has not yet had an opportunity 



