IS4 



produce a crystalline cleavage through whole mountain chains of me- 

 chanical rocks ; 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 

 granite are considered as examples of this structure. A jointed struc- 

 ture of this kind may in some instances be derived from an original 

 globular structure ; but the ])rismatic and cuboidal blocks of granite 

 are not considered as due to such a cause, and the concentric crusts 

 into which such blocks are found to decompose, are regarded as the 

 natural effects of decomposition on a mass of homogeneous structure. 

 This conclusion is supported by the fact, that artificial pillars of gra- 

 nite (or even of oolitic limestone) sometimes decompose in concentric 

 cylindrical crusts. 



In the preceding cases a jointed structure is, both in its origin and 

 in the mineral phaenomena it presents, entirely distinguished from a 

 slaty cleavage. Some granitic rocks (without a vestige of true bed- 

 ding) have, however, an imperfect cleavage. The granite of St. Aus- 

 tell Moor is described as made up of highly inclined parallel lamina- 

 tions ranging about magnetic east and west 3 and on some parts of 

 the region, the laminations, on approaching the schistose rocks, are 

 extremely fine, and seem to form a passage between the killas and 

 the granite. Such an appearance is, however, the exception, and not 

 the rule. Again, the prismatic joints of the granite sometimes par- 

 tially affect the neighbouring slate rocks. But facts like these only 

 prove that the granite and the contiguous schists passed into their 



