] 94: PROF. T. G. BONNEY ON CRYSTALLINE SCHISTS AND THEIR 



from its present condition. There are, however, certain rocks out- 

 cropping higher up the hill, obviously in close relation with tlie 

 above-mentioned marble, which demand careful study ; for at first 

 sight these might be supposed to retain traces of organisms. Some 

 300 feet above the level of the valley, we found that the marble 

 (here rather more fissile and flaggy) shows on the southern side elon- 

 gated markings of a lead-colour, bearing a rough resemblance to 

 flattened tubes. These vary in size ; the larger are a couple of inches 

 or even more in length, a quarter of an inch wide, and perhaps one 

 twentieth of the same in general thickness. They weather more 

 rapidly than the rest of the rock, so as to be indicated on an old sur- 

 face by irregular hollows. Higher up, about on the same horizon, 

 the rock is obviously not quite so pure a limestone (having, at first 

 sight, as is common with such rocks, a slightly gneissoid aspect), and 

 is more distinctly " slabby " in habit. In this variety also the rudely 

 tubular markings are very distinct ; they are lighter in colour than 

 the body of the rock (which here is greyish), and are occupied by a 

 substance resembling a yellowish-white or greyish clay. 



Specimens of these rocks have been studied microscopically, more 

 especially with a view of determining the nature of these peculiar 

 structures. The first one, with the lead-coloured streaks, is a fairly 

 coarsely crystalline limestone (grains commonly about '03" in dia- 

 meter) with a few flakes of white mica, grains (variable in size, but 

 generally smaller than those of calcite) of quartz *, and a few black 

 grains, probably graphite. The matrix does not appear to be gene- 

 rally crushed, but some grains exhibit the usual indications of pres- 

 sure. The " tubes " (cut transversely in this specimen) are occupied 

 by minutely granular calcite with occasionally a small flake of mica, 

 which mineral seems slightly more abundant here. The sections 

 are irregular in outline. They are streak-like masses, which have 

 a tendency to die away in a string of granules ; now and then a 

 larger grain of calcite occurs in the dust-like mass, and the grains 

 of the matrix on either side of the streak seem as if broken away. 

 These streaks obviously lie, whatever be their significance, in sur- 

 faces roughly at right angles to the general direction of the pressure 

 by which the rock has been aftected. A specimen of the rock with 

 light-coloured tubuli, in close relation with the last described, though 

 a little further away from the more typical marble, does not exhibit 

 any important difference under the microscope, but the grains of 

 calcite are more dirty-looking. Sections of the tubuli are numerous, 

 often very irregular and " inorganic " in outline, sometimes darkened 

 as if by graphitic matter, either crushed up in situ or subsequently 

 infiltered. Another specimen, taken from higher up the section, 

 obviously less calcareous, consists of calcite as before, quartz, in 

 amount perhaps one third of the whole, with a little white mica 

 and granules of graphite (?). Sections of the tubuli are numerous ; 

 they are very irregular in outline, " streaking " away into the ma- 

 trix, and small ones occur which are hardly more than strings of 



* I think it possible that a colourless silicate may also be present. Not sel- 

 dom crystallites are enclosed. 



