96 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, 



general aspect which these jointings assume. Considerable modifica- 

 tions sometimes take place. The E. and "W. oblique courses in some 

 cases incline at a greater, and sometimes at a less, angle ; and the 

 horizontal series has its place supplied by joints which are also more 

 or less inclined. There is, however, in all these modifications of 

 direction a great tendency of the joints so to arrange themselves 

 mutually as to cut the masses included between the divisional planes 

 into rhombs having their acute angle 45°. The form of the angles 

 which result from joints, bear greater affinity to those which crystal- 

 lized carbonate of Hme assumes, and would, to some extent, support 

 the inference, that, while the vertical N. and S. joints which intersect 

 the limestones (with their various chemical compositions) are common 

 to them and the underlying arenaceous Devonian beds, and must 

 have resulted from the same causes, the oblique and horizontal joints 

 (and their modifications, which appertain only to the purer limestones) 

 are the products of another cause ; and this cause would seem to be 

 that which produces crystalline arrangement in carbonate of lime. 

 There are two circumstances, however, which seem hostile to this 

 inference : — one is the finely- developed jointing which manifests 

 itself in the drab shales of Clunnie quarry, already referred to, where 

 the chemical nature is far from homogeneous, and where no crystalline 

 arrangement could result; the other is the limestones themselves 

 possessing none of that cleavage which is an important feature in 

 crystallized carbonate of lime. 



Fossils affected hy the Jointings. — There is another circumstance in 

 connection with the jointing of the limestones which requires notice. 

 This is, the condition of the fossils which occur in these deposits. 

 Where we have the N. and S. joints well developed and closely ap- 

 proximated, and where fossils make their appearance in the limestones 

 intersected by these joints, the fossils are very commonly more or 

 less distorted. To such an extent does this prevail, that it is rare to 

 find any species of the most common gasteropod, Euomphalus, in its 

 normal condition. Almost every individual has an excentric form ; 

 and this deviation from the normal condition occurs likewise in 

 brachiopods. Spirifera striata, the most common of these, is usually 

 much distorted, and in some cases as far removed from its original 

 state as any specimen of this shell which may be distorted by slaty 

 cleavage. 



Observations on these distorted shells in situ lead to the inference 

 that the distortion has taken place between the jointings which have 

 the IsT. and S. courses, — the shells being elongated towards the divi- 

 sional planes, and flattened at right angles to these planes. 



In other words, the elongation has been in an east and west direc- 

 tion — or in what would have been the strike of the slaty cleavage, 

 had this structure affected these rocks, which it does not — and not 

 in the direction of the dip of the cleavage-planes, the mode in which 

 such fossils are elongated which have been subjected to distortion 

 from slaty cleavage. 



Age of the Jointings. — With regard to the subject of the relative 

 age of the joints which affect the deposits in this locality, the only 



