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ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 829 
other non-isotropic rocks and minerals ; a method depending on the fact, 
that a cleaved rock conducts heat better along than across the cleavage- 
planes. His process is to cut a section perpendicular to the cleavage, 
coat the surface with a thin film of grease, and heat at one point. It is 
found that the curve marked out by the ridge of melted grease after its 
cooling is not a circle, but an ellipse whose longer axis is along the trace 
of the cleavage. The ratio of the axes of this ellipse indicates the relative 
thermal conductivity of the rock along and across the cleavage-planes, 
and may be regarded as, to some extent, an index of the non-isotropic 
character of the rock.! In the Grenoble section this ratio was found to be 
greatest in those beds which contained most argillaceous and least cal- 
careous matter, as appears from the table quoted below. 
Percentage composition 3 : 
5 I Ratio of axes of ellipse 
of conductivity 
Argillaceous matter | Calcareous matter 
62 38 1:04 
35 65 1:23 
6 94 11 
10 90 1:06 
Sandstones, for the most part, are capable of taking but a very rude 
cleavage. In Wales the diabase dykes, so common in some slate-quarries, 
occasionally exhibit the same structure in a very imperfect degree. 
When alternating beds of different lithological characters have been 
subjected alike to the forces which produce cleavage, it frequently 
happens, as was long ago noticed,” that the more fine-grained and 
argillaceous beds have acquired a cleavage-structure, while the coarser 
beds, calcareous or arenaceous, have not been so affected. Even when 
intercalated sandy beds among slate rocks have received a certain 
cleavage structure, it is not only of a more rudimentary character than 
that of the slate-rock, but has a slightly different angle of dip, the 
cleavage surfaces being bent or curved at the junction of the two kinds of 
rocks. Most writers on the subject of cleavage have described instances 
of these phenomena. In the Welsh slate quarries the ‘steps’ produced in 
the cleavage planes by the thin gritty bands of rock are very noticeable. 
Where the slate rock rests upon a gritty band, the dip of the cleavage is 
seen to change abruptly on passing from the one rock to the other, the 
deviation sometimes amounting to as much as 20° or 30° (fig. 7). On 
emerging into the slate rock again, the cleavage planes resume their 
original direction in a manner suggestive of the refraction of a ray of light 
through a plate of glass. If gritty bands of different textures occur 
? This ellipse is not, however, the same as the trace of the strain ellipsoid; its 
axes are in the same directions as those of the latter, but less unequal. For example, 
in some slates from Nantlle and Groeslon, I find for the ratio of the axes of the ellipse 
of conductivity, by three experiments, the numbers 1:23, 1:24, 1:21; the ratio of the 
axes of the strain ellipse is much greater. Cf. Dufet, Ann. de Ecole Norm. Sup., 
sér. 2, t. iv. p. 185 et seg. (1875). 
M. Jannettaz has also shown that the conductivity in the direction of cleavage-dip 
is slightly greater than that in the direction of cleavage-strike, which is in accord- 
ance with what might be expected. 
, * £.g., De la Beche, Geological Observer, p. 616, 2nd ed. (1853). Geikie’s Zeatbook 
of Geology, p. 311, figs. 75, 76 (1882). 
