TRANSACTIONS OF THE SECTIONS. 9^ 



curves of the bedding would have been very diflferent from wrhat may be seen by 

 examining the rocks. 



In the district about Loch Lomond the beds have been so bent by elevation that 

 the clay-slate is newer than the metamorphic rocks in immediate contact with it. 

 The dip of cleavage follows a uniform law in both, and shows that the elevating 

 force there was on the north side, as is also indicated by the bedding. 



On some of the Mechanical Structures of Limestones. 

 By H. C. SoRBY, F.G.S. 



The author considers that the only satisfactory method of ascertaining the true 

 structure of limestones, is to examine thin sections of them with the microscope. 

 The results described in this paper were arrived at in this manner. Limestones 

 have been usually described as more or less crystalline or earthy, but this has re- 

 ference chiefly to subsequent changes, and not to their original condition. When 

 examined with the microscope, it is seen that to describe them according to their 

 mechanical characters would usually be far better. In this manner they may be 

 very conveniently classed as organic sands or clays ; in the same way that we may 

 speak of felspar, sand or clay. The organic structure of the minute fragments of 

 which they are composed is often so well preserved, that their nature and relative 

 proportion can be satisfactorily determined. 



Where they have been consolidated, the shrinking of the mass has often produced 

 cracks and joints, afterwards filled with calcareous spar, and often presenting a 

 beautiful appearance, when examined with the microscope, on account of their 

 number and regularity ; and showing faults of ^oth of an inch, or much less. 

 These are totally distinct from slaty cleavage, which can be studied to great advan- 

 tage in such limestones as have that structure. The author has proposed a theory 

 to account for this, and has shown that the rocks that possess it have been so 

 much compressed, as shown by a great variety of facts, that the position of their 

 ultimate particles would be changed in such a manner as to give rise to precisely 

 such a structure as that which produces cleavage. That this would be a necessary 

 result may be proved both by calculation and experiment. In the case of lime- 

 stones, it is impossible to suppose that any other than a mechanical cause can have 

 developed the structure seen with the microscope, because the particles whose posi- 

 tion has been changed are fragments of organic bodies, and not crystals. Besides 

 this change of position, in many cases minute organic fragments, whose original 

 form and structure are well known, are greatly compressed in the plane of cleavage, 

 as shown by the change in their form and structure ; and even crystals of dolomite 

 are broken up, elongated, and their crystalline cleavage planes bent ; thus showing 

 that the rock was in a consolidated condition when the change of dimensions 

 occurred, but that the pressure was so intense, and acted so gradually, that the 

 whole mass of rock gave way like more malleable substances, by the movement of 

 the particles one over another. 



On the Currents produced by the action of the wind and tides, and the struc- 

 ticres generated in the deposits formed under their influence, by which the 

 physical geography of the Seas at various geological epochs may be ascer- 

 tained. By U. C. SoRBY, F.G.S. 



The first division of this communication consisted of a description of the nature 

 and peculiarities of the currents produced at the present period by the operation of 

 the tides, waves, and winds, and their relations to the physical geography of the 

 sea, in order that a proper judgment might be formed with reference to those of 

 former epochs. It is thus seen that a knowledge of the directions and characters 

 of the currents would furnish very much information respecting the general physical 

 peculiarities of the seas, and the position and direction of their coasts. 



The second division comprised an account of observations and experiments re- 

 specting the effects of currents on the deposits formed under their influence, by 

 ■ 1855. 7 



