CLEAVAGE AND BEDDING. 1 3 



must be taken to discriminate between this and * Sla/y cleavage,^ 

 which often intersects the Palaeozoic strata at all angles, and is a 

 phenomenon produced by "viscous shearing" or by great lateral 

 pressure subsequent to the consolidation of the beds.^ Jukes has 

 observed that if the beds undulate while the dip of the cleavage is 

 steady, and if the substance of the slate be very homogeneous, 

 it may easily happen that the stratification is only well shown when 

 it has a given relation to the cleavage ; when, for instance, they 

 coincide, or when they are at right angles to each other, or when 

 they cross at some other angle, so that the particular mark of 

 stratification which the kind of slate possesses shall be least 

 obscured by the cleavage. 



To determine the true dip when the beds are affected by 

 ' cleavage,' lines of organic remains, and flaggy or gritty bands 

 must be looked for; sometimes this may be done by evidence 

 of what Sedgwick called the ' stripe,' which consists of bands of 

 different colour, whether grey, purple, or green, which run through 

 the slaty rocks and indicate different layers of mineral matter.'^ In 

 rare cases, disturbances seem to have contorted the cleavage planes. 



Bedding is the term applied to marked lines of stratification 

 which separate the rock into more or less distinct layers from a few 

 inches to a foot and more in thickness, and probably indicate 

 slight pauses in deposition. Lamination, which is most conspi- 

 cuous in clays and shales, is applied to the splitting of rocks into 

 thin films, which are in many cases the original layers of depo- 

 sition. Paper-shale is ver}^ finely laminated. In some cases we 

 find rapid alternations of sand and clay. Joints are produced by 

 shrinkage, and sometimes by disturbance. They are cracks which 

 occur generally at right angles to the bedding ; and when enlarged 

 by the action of water, they are converted into fissures. Sometimes 

 joints may occur as irregular divisional planes, simulating false- 

 bedding. These "Rift -joints" may also be due to contraction and 

 shrinkage.^ 



False-bedding ox Oblique lamination is a feature produced in shallow 

 water by currents and tidal action, whereby beds are heaped up in 

 irregular layers without any approach to horizontality or continuity. 

 Minute false-bedding is seen in Sandstones and Oolites ; and more 

 conspicuous wedge-shaped bedding is often seen in the same 

 rocks. 



The term Foliation is applied to the arrangement of mineral 

 matter in alternate layers of different composition ; and is cha- 

 racteristic of metamorphic rocks. (Seep. 5.) 



Terminal curvatiwe is applied to the local and superficial disturb- 

 ance of shaly and slaty rocks whereby the beds (or their cleavage 

 planes) are bent over and present an opposite direction in dip to 

 the beds below. In some instances it is traceable to the action 01 



1 See O. Fisher, G. Mag. 18S4, p. 276, 18S5, p. 174. 



2 T. G. S. (2), iii. 473. 



3 See J. G. Goodcliild, G. Mag. 1S83, p. 39S ; and H. C. Sorby, Address to 

 Geol. Soc. 18S0. 



