ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 815 
may cut the bedding at any angle, according to the varying dips and 
-contortions of the latter ; he stated that they are much more regular in 
their arrangement than the bedding, maintaining their straight course 
throughout large tracts of country, and their dip changing very gradu- 
-ally as we cross successive planes; finally he laid down, as the result of 
his own researches in districts of slate-rocks, the important law, that 
“where the cleavage is well developed in a thick mass of slate-rock, the 
strike of the cleavage is nearly coincident with the strike of the beds.’ ! 
This last statement was modified by Professor Phillips, Dr. Darwin,? 
and Mr. Sharpe, who replaced it by the law, that the strike of the cleay- 
age planes in a district is ‘ parallel to the main axes of elevation,’ and not 
necessarily with the strike of the beds at any given locality. In this 
form the law was borne out by the researches of field-geologists ? in all 
parts of the world where slate-rocks are met with. The law as enunci- 
ated by Professor Sedgwick has many exceptions, as he himself was the 
first to remark. 
The alleged second direction of cleavage in certain slates, at right 
angles to the chief cleavage, as stated by Mr. D. Sharpe,‘ and subse- 
quently by Professor Sedgwick® and others, is a point to be discussed 
below. 
The next advance was the observation that the fossils associated with 
cleaved rocks are commonly more or less distorted in form, and that the 
mode of distortion is related to the direction of the cleavage-planes in the 
rock. This fact was first brought into notice by Professor Phillips,® but 
its significance was only subsequently made clear by Mr. Sharpe.? The 
latter geologist drew the conclusion that rocks affected by slaty cleavage 
have suffered a compression of their mass in a direction everywhere per- 
pendicular to the plane of cleavage, and an expansion in the direction of 
¢leavage-dip. The numerical results of Professor Haughton,* merely 
mentioned by Professor Phillips in an addendum, confirmed at least the 
former of these conclusions. 
Messrs. Sharpe® and Sorby’? showed that the form and arrangement 
of the fragments, both macro- and micro-scopic, which compose the mass 
of a cleaved rock, agree in every respect with the kind of distortion 
‘supposed, and made various other observations having the same bearing. 
Finally, as regards the theories of slaty cleavage, Bakewell! and most 
of the earlier investigators considered the structure to be ‘ the effect of 
crystallisation’; MacCulloch!* supposed it to be the result of ‘ concre- 
tionary action’; Professor Sedgwick! himself ascribed it to ‘ crystalline 
1 Loc. cit. p. 473. ? Geological Observations in South America, p. 163, 1846. 
* Darwin, Forbes, Harkness, Hopkins, Jukes, Murchison, Phillips, Ramsay, Rogers, 
Sedgwick, Sharpe, &c. * Quart. Journ. Geol. Soc., vol. v. p. 114, 1849. 
° Synopsis of British Paleozoic Rocks, Introd. p. xxxy., 1855. London and 
Cambridge. 
° Brit. Assoc. Rep., 1843, Trans. sec. p. 61. 
7 Quart. Journ. Geol. Soc., vol. iii. p. 74, 1847. 
8 Phil. Mag. 4th ser., vol. xii. p. 400 (1856). 
° Quart. Jowrn. Geol. Soc., vol. v. p. 112 (1849). 
%° Edinb. New Phil. Jowrn., vol. lv. p. 137 (1853). Phil. Mag., 4th ser., vol. xi. 
'p. 20 (1856). 
1 Introduction to Geology, p. 86 (1813). 
1 Western Isles of Scotland, vol. iii. p. 33 (1819). Journ. Roy. Inst. (1825). 
System of Geology, vol. ii. (1831). 
18 Geol. Trans., 2nd ser., vol. iii. p. 477 (1835). Synops. Brit. Pal. Rocks, Introd. 
(1855). Cf. Professor H. D. Rogers, Trans. Roy. Soc. Edinb., vol. xxi. p. 464 (1856). 
. 
