TRANSACTIONS OP THE SECTIONS. 77 



Description of a Working Model to illustrate the formation of " Drift-bedding " 

 (a kind of false stratification). By H. C. Sorby, F.G.S. 



This model was constructed to explain the manner in -which that kind of false 

 stratification, for which the author has proposed the term " drift-hedding," is produced 

 by the sandy material being drifted along on the bottom, till the depth of the water 

 becomes so much greater, that the velocity of the current is not sufficient to wash it 

 any farther. It then accumulates in stratula, inclined to the horizontal plane at angles, 

 the value of which depends upon various circumstances. In the model, the drifting 

 effect of the current was intimated by a kind of coarse screw, which, when turned 

 round, carried forward the sand, supplied from a bag, along a groove, from which it 

 fell into a space with a glass front, where it accumulated at the angle of rest. Being 

 a mixture of heavy black fine grains of specular iron and coarser white quartz sand, 

 it became sorted by moving the screw alternately quickly and slowly, and thus accu- 

 mulated in black and white bands ; whereas, if it was moved with a uniform velocity, 

 no such bands were produced, but the coarse white particles collected at the bottom. 

 These effects, thus produced experimentally by an irregular or uniform forward 

 moving action of the screw, are precisely the same as what the author had previously 

 deduced to have been generated in strata of various geological periods by currents of 

 varying velocity ; and the appearance of the structure, thus formed in the model, so 

 closely agrees with what is so commonly met with in sandy rocks, that no one can 

 doubt how it originated. Such models may now be procured of Messrs. Chadburn 

 Brothers, Sheffield. 



On the Magnesian Limestone having been formed by the alteration of an ordi- 

 nary calcareous deposit. By H. C. Sorbt, F.G.S. 



It is well known that crystals of calcareous spar are in some cases found changed 

 into dolomite, and that corals and other calcareous organisms are often altered in a 

 similar manner, and their organic structure obliterated. It is therefore clearly 

 proved that such a change may take place in calcareous rocks. Portions of the car- 

 boniferous and Devonian limestones have also frequently experienced this change, 

 and it has so taken place along joints and veins, that no explanation appears probable, 

 but the long-continued action of some soluble magnesian salt. 



When thin sections of such rocks are examined with the microscope, some trace of 

 the fragments of organic bodies of which they were composed may be seen in some 

 cases, but in many the original mechanical structure has been entirely obliterated by 

 the change, and there is now only a peculiar crystalline structure, chiefly due to the 

 more or less interfering action of minute rhombohedrons. The same is seen in thin 

 sections of the Permian dolomite ; so that a considerable portion, if not the whole, 

 appears, like other limestones, to have been derived from comminuted and decayed 

 calcareous organisms, and to have been subsequently altered into dolomite. If such be 

 the case, the author suggested that probably this alteration was effected by the infiltra- 

 tion of the soluble magnesian salts of the sea-water, under some peculiar conditions not 

 yet clearly explained, during the period when it became so far concentrated that rock- 

 salt was frequently deposited ; and that the calcareous salt removed during the change 

 had, by decomposition with the sulphatesof the sea-water, given rise to the accumula- 

 tions of gypsum. In support of this, it is an important fact, that some very solid dolo- 

 mite does even now still contain about one-fifth per cent, of salts soluble in water, con- 

 sisting of the chloride.i of sodium, magnesium, potassium and calcium, and sulphate of 

 lime, doubtless retained in the minute fluid cavities, seen with the microscope to exist 

 in great numbers. These, like those in most crystals formed from solution, must have 

 been produced at the same time as the dolomite, and caught in some of the solution 

 then present, which is thus indicated to have been of a briny character. 



A process the very reverse of that just described is now taking place by the action 

 of dissolved gypsum, by which sulphate of magnesia, frequently efflorescing on the 

 surface of the rock, and carbonate of lime are produced ; and this may perhaps, in 

 some cases, explain why the upper beds of the Permian limestone are now more cal- 

 careous than the lower. 



