818 REPORT—1885. 
clusions entirely in agreement with those of the geologist just quoted. It 
appears from his observations that a common form for the strain ellipsoid 
in the slates of North Wales is one whose axes are in such ratios as 
16:1:027. These figures indicate a total diminution of volume in the 
ratio 0°43 : 1. 
Professor S. Haughton! made numerical calculations of the distortion 
of form exhibited by the fossils of cleaved rocks, examining specimens 
from eight localities in Ireland, Cornwall, and Wales. He found a very 
marked compression in the direction perpendicular to the cleavage-planes, 
but his results led him to reject Mr. Sharpe’s other laws relating to the 
expansion along the cleavage-dip and the unchanged dimensions along 
the cleavage-strike. His ellipsoid of strain was thus an oblate spheroid 
or ellipsoid of revolution, having its equator in the cleavage-plane. This 
implies, either no expansion in any direction, or equal expansion in all 
directions along the cleavage-planes. In the former case the diminution 
of volume would be in the ratio c : b (orc: a) ; in the latter the diminu- 
tion of volume, if any, would be indeterminate by this method. Professor 
Haughton’s results, expressed in the notation already employed, are given 
in the following table :— 
Ratios of axes of ellipsoid 
Carboniferous slate of Ardroginna, Waterford . . | 0°975 3 1:000 : 0°412 
1 
2 5 5 South Petherwin, Cornwall . | 1°010 : 1:000 : 0:256 
3 - “ Tintagel, Cornwall . . | 0°669 : 1000 : 0°102 
4 Lingula beds of Abereiddy Bay, Pembrokeshire ./| 1-000: 1:000 : 0-145 
5 Green grits of Llyn Padarn, Llanberis. . - | 0°805 : 1:000 : 0°531 
6 Silurian black slates of Moel Benddu, North Wales. | 1:000 : 1:000 : 0-270 
i = > Garth, Portmadoc ; . | 1:000 : 1-000 : 0-090 
8 Carboniferous slate of Carrigaline, Co. Cork . - | 1:000 : 1:000 : 0°466 
In the last three examples it is asswmed that a and b are equal, and ¢ 
is calculated on this assumption, which seems scarcely warranted by the 
results obtained for the first five rocks examined. It will be noticed that 
Professor Haughton’s investigations included the fossils on which Mr. 
Sharpe based his conclusions, but the more perfectly cleaved roofing 
slates, such as those of North Wales, studied by Dr. Sorby, are excluded, 
owing to the absence of fossils in them. 
Professor J. Phillips,? in an early paper read before the British 
Association, described the distortion of cleaved rocks as a ‘creeping 
movement among the particles of the rock, along the plane of cleavage, 
the effect of which was to roll them forward, in a direction always 
uniform over the same tract of country.’ This rather indefinite language 
may be interpreted to mean what would now be termed a shearing 
motion * along the cleavage planes. In other words, if we conceive the 
rock divided into indefinitely thin slices, parallel to the cleavage planes, 
then each slice is supposed to have slipped an indefinitely small distance 
over its neighbour in the direction of the cleavage dip; the relative dis- 
1 «On Slaty Cleavage and the Distortion of Fossils,’ Phil. Mag., vol. xii. p. 409 
(1856). ‘Ona Model Illustrative of Slaty Cleavage,’ Brit. Assoc. Rep., 1857, Trans. 
sect. p. 69. 
7 ©On Certain Movements in the Parts of Stratified Rocks,’ Brit. Assoc. Rep., 
1843, Trans. sects. p. 61. 
* Thomson and Tait, Natwral Philosophy, vol. i. pt. i. §§ 169-171, new ed. (1879). 
Minchin, Treatise on Statics, p. 474, new ed. (1880), Oxford. 
