Geological Society. 323 



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 prismatic 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 phenomena 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 $ 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 

 ultimate solid state at the same time, and under similar conditions, 

 and throw no difficulties whatever in the way of the igneous theory 

 of granite. Some writers, by confounding such joints with beds of 

 deposit (to which they bear no analogy), have been led into most pre- 

 posterous conclusions. 



Many of the great parallel veins of St. Austell Moor are described 

 as veins of segregation j yet some of them are metalliferous. Most 

 of the metalliferous veins and cross courses of Cornwall are, however, 

 considered by the author as veins produced by mechanical disruption -, 

 but on this hypothesis, the direction of such veins would (in part at 

 least) be affected by the structure and direction of the rock: and 

 from these considerations he thinks that we might (independently of 

 any direct observations) expect in some parts of the county a system 

 of veins running nearly magnetic east and west, and a great system 

 of cross-joints nearly at right angles to that direction. 



Before concluding the paper, he briefly notices the principal direc- 

 tions of the great cross courses and veins traversing the mountain 

 limestone of Derbyshire and Flintshire. The cross courses are nearly 

 in the direction of the beds, the veins nearly at right angles to them : 

 and these two directions harmonize very exactly with the theory which 

 refers both sets of fractures to a mere mechanical disruption of the 

 rock. 



April 8. — A paper was first read, entitled " Notice on the Junc- 

 tion of the Portland and the Purbeck Strata on the Coast of Dorset- 

 shire ;" by William Henry Fitton, M.D., F.R.S., &c. 



The occurrence of silicified trunks of trees in the upright position, 

 with their roots in a thin bed of carbonaceous clay and coarse gravel, 

 a few feet above the top of the oolite, in the Isle of Portland, was 



2T2 



