96 REPORT — 1855. 



On New Forms of Crustacea from the District of Lesmahagow. 

 By R. Slimon*. 



On the Shelly Deposits of the Basin of the Clyde, with proofs of change of 

 Climate. By James Smith, F.R.S., F.G.S. 



On the Strticture and Mutual Relationships of the older Rocks of the High' 

 land Border. By H. C. Sorby, F.G.S. 



The author first gave a short outUne of the opinions that have been entertained 

 by various geologists respecting the origin of that structure in the metamorphic 

 rocks, for which the term foliation has been proposed. He much objects to this, if 

 used as though there were but one structure present in them ; for, by a careful and 

 close inspection, with or without the microscope, two, that are most distinct from 

 one another, may very often be recognized. One of these has every character that 

 would be the result of stratification, even in some cases including the current struc- 

 tures ; and the other is related to it in precisely the same manner that the cleavage 

 of slate rocks is to their bedding. This is best seen in the more micaceous bands 

 in contorted beds of gneiss, and in them the crystalline flakes of mica often lie, not 

 in the plane of the bands themselves, but pass on in one uniform direction, whilst 

 the bands of varying composition bend about and are often perpendicular to the 

 general direction of the flakes of mica. One of the most decided relations between 

 cleavage and bedding is that cleavage lies in a plane perpendicular to the line in which 

 pressure has acted, so as to change the dimensions of the rock. The structure just 

 alluded to, as in some districts affecting mica-schist and gneiss, agrees with it in 

 this ; and in fact presents us with all the peculiarities that could be expected from 

 metamorphosed cleavage, in the same manner as the other does with respect to 

 stratification. 



In some districts it appears to be absent, as is also the case with cleavage, 

 and then only that analogous to stratification is to be seen. Metamorphic rocks 

 are often very full of contortions, some larger and others quite small. These appear 

 to have been formed in various manners ; but may be accounted for on strictly me- 

 chanical principles. In order to explain this, the author had constructed models to 

 represent beds that could readily and evenly give way and change their dimensions 

 when elevated or bent, and others that would not admit of this ; and by bending or 

 elevating them, in the manner that is seen to have occurred in the case of the rocks, 

 when composed of elastic material, no contortions are produced; whereas, in the 

 other case, they are formed, and have precisely the same relations to the character 

 of the elevation or bending, as those met with in the rocks themselves. The author 

 therefore is of opinion, that nearly the whole of them may be explained on strictly 

 physical principles, by supposing that mica-schist and gneiss were in a more or less 

 softened condition when the movements of elevation occurred, and not in a state 

 analogous to the unaltered rocks, that have yielded to similar actions in a very dif- 

 ferent manner; and this supposition he thinks would agree with what is indicated 

 by other facts. 



In carrying out these inquiries, some sections of the Highland border had been 

 constructed, in which the structure described above as due to stratification was 

 carefully distinguished from that considered to be produced by cleavage ; and the 

 result is, that there is every reason to believe that the clay-slate rocks are not more 

 recent than the whole of the mica-schist, as has been supposed, but are older than a 

 considerable portion of it; and on the whole are the same group of rocks prolonged 

 to a distance beyond the limit of the metamorphic action. This supposition com- 

 pletely explains all the peculiarities observed; whereas, if their dip under the altered 

 rocks was only apparent, and due to inversion, there is very good cause for conclu. 

 ding that the relations of the cleavage and the axes of the contortions to the general 



* The tract has since been examined by Sir R. Murchison and Professor Bamsay, and is 

 described in the Quart. Journ. Geological Society, March, 1856 ; the crustaceans being 

 described by Mr. Salter. 



