A. Bravunall — Some llicrocJieonical Methods. 125 



On comparison of these preparations it is seen that (i) chalybite — - 

 often partly oxidized to limonite — may be located in a narrow 

 peripheral zone of an oolith or in roughly concentric zones forming 

 the greater part of the oolith, or (ii) chalybite may compose the entire 

 structure. A nuclear mosaic of calcium carbonate granules may be 

 the residue of an original aragonite oolith/ which is undergoing 

 metasomatic replacement by chalybite, the latter being usually 

 present in the matrix. 



2. In the investigation of minute structures — stream lines and 

 channels — in certain ottrelite-bearing rocks it was desired to 

 determine whether the mineral matter infilling these channels 

 contained an appreciable quantity of manganese. The test applied 

 is described below. Such structures are very minute, and the results 

 of the test require close scrutiny, otherwise significant spots may be 

 overlooked and the reaction may appear too vague to be of much 

 value. 



A slice is ground down until it is about the thickness of a florin, 

 and one side is polished. The slice is detached from the glass slip, 

 cleansed of balsam, and dried (at about 150 degrees to desiccate the 

 shce, and, if possible, promote fissures along the channels) ; it is 

 then allowed to soak for several minutes in a hot saturated solution 

 of K2CO3 + KNO3, and dried. A thin wash of the hot solution 

 is applied to the polished surface, the slice is again dried and laid 

 on a tile, polished surface uppermost ; this surface is subjected to 

 the F.P. tip of the blowpipe flame for a few minutes, and when 

 cold it is slightly polished to level (not to remove wholly) the thin 

 veneer of " melt ". The slice is then mounted (prepared surface 

 downwards) in balsam, ground down as thin as possible, and 

 examined by transmitted light. Before the blowpipe a slice which 

 is too thin tends to buckle, and it may be necessary to break it up, 

 after treatment, into two or more flakes to be mounted separately. 



3. Bedding planes in chalk and limestone are often in- 

 distinguishable ; nevertheless, extremely thin laminae may be 

 relatively rich in foreign matter (volcanic or meteoritic dust, for 

 example), and these may be brought to light by the following 

 method — approximating to a process of weathering : A specimen 

 of the rock is ground evenly and polished in one or more selected 

 directions, washed, and immersed in a bath of carbonated water. 

 By differential solution, controlled by lack of homogeneity, bedding- 

 planes, minute structures, etc., may be revealed. (Foraminifera 

 rarely escape solution unless the water is very feebly carbonated ; 

 the process is then extremely slow.) The bath should be rocked 

 occasionally, but very gently, otherwise deUcate and significant 

 structures may be obliterated. 



1 Rastall, Nature, July -3, 1919, p. 359. 



