322 Geological Society. 



slate quarries there is no indication whatsoever of the true bedding; 

 but whenever the slates have a striped structure, the stripes (so well 

 known in the Cumberland and Welsh roofing-slates) are parallel to 

 the true beds. To this rule there is no exception in the regions de- 

 scribed 5 and in thousands of instances the stripes are seen to be 

 parallel to the alternating masses of 'coarse greywacke^, and to cal- 

 careous beds with organic remains. 



The author then describes a flaggy, passing into a finely laminated, 

 structure, parallel to the bedding. He points out the manner of di- 

 stinguishing this from a true slaty structure, which may readily be 

 done in a quarry, and, generally, even in examining hand specimens. 



In this view, a laminated structure and a slaty structure differ so 

 entirely in their origin, that however nearly they may resemble each 

 other in some instances, they ought never to be confounded under 

 the same name. 



Finally, he distinguishes cleavage planes from the contorted laminae 

 of argillaceous schists ; and endeavours to prove, by a long series of 

 sections derived from various parts of North and South Wales, that 

 the introduction of a crystalline cleavage was the last chemical change 

 superinduced on the slaty deposits before they became entirely solid. 



He then speculates on the enormous amount of force necessary to 

 produce a crystalline cleavage through whole mountain chains of me- 

 chanical rocks j and supposes it due to an accumulated intensity of 

 crystalline action in a nearly homogeneous mass, every part of which 

 is exposed to the same conditions of aggregation while passing into 

 its ultimate solid form. He illustrates this principle by contrasting 

 the structure of the enormous calcareous deposits of the Alps with the 

 structure of the thin interrupted limestone formations of England. 



The foliated uneven layers of old, crystalline schists (such as chlorite 

 schist, and mica schist) are briefly noticed, and considered generally 

 to form portions of beds, and not of cleavage planes : but to this rule 

 he gives some remarkable exceptions. 



In all slate rocks, besides the cleavage planes, there are found one 

 or more sets of cross-joints, which often separate the rock into regular 

 rhombohedral solids. Even in hand specimens of such solids we may 

 detect which is the cleavage plane, because, parallel to that plane 

 (and to that plane only) the mass admits of indefinite subdivisions. 

 The direction of one set of joints is generally inclined at a great angle 

 to the direction of the beds : and, hence, as the prevailing strike of 

 the slate rocks of England is north-east, we may expect the prevailing 

 strike of one set of joints to be nearly north-west. 



This portion of the paper concludes by recommending a more con . 

 sistent use of technical language in the description of slate rocks than 

 is commonly met with in the published works on this part of geology. 



§ 4. Jointed Structure. 



Rocks, both aqueous and igneous, have undergone a mechanical 

 tension while passing into a solid form ; and, in consequence, many 

 of them have become subdivided by a number of parallel fissures, pro- 

 ducing a jointed structure. Jointed pillars of basalt and prismatic 



