378 Proceedings of the British Association. 



on the alternation of beds of different mineral characters, and on 

 the circumstances observed with regard to the inclination of layers. 

 He stated, from his own observations, and referred to a recent pub- 

 lication for detailed descriptions, the fact, that all the characters 

 usually considered as characteristic of stratification in primary slates, 

 do also occur in granites, and that the essential structural charac- 

 ters of these slates are continued into the neighbouring granites, 

 thence inferring, that no real structural distinction existed between 

 the granites and the primary slates. 



Professor Sedgwick entered on the question at considerable 

 length, and stated that, sixteen years ago, after a visit to Cornwall, 

 he had been led to adopt the opinions now held by Dr Boase, but 

 that his subsequent experience, and more especially his investiga- 

 tions in North Wales and Cumberland, had produced a considera- 

 ble change in his views. He discussed the principles upon which 

 questions of this nature ought to be considered, and particularly 

 stated the impossibility of giving definitions which w ould be ap- 

 plicable to all cases. The Professor stated as his belief, that it is 

 impossible to separate the lower and higher parts of the slate se- 

 ries ; and is of opinion, that it is generally quite practicable to dis- 

 tinguish between true stratification and laminar structure. He 

 mentioned particularly the assistance to be derived from the striped 

 appearance so common in the slates of Wales, and noticed the dis- 

 tinctions between certain structures of rocks, and the characteristic 

 marks of true stratification. Professor Sedgwick expressed it as 

 his opinion, that the laminar structure has been produced at a pe- 

 riod subsequent to that of the formation of the slaty rocks in which 

 it occurs. 



Mr Greenough directed the attention of the Section to cases 

 where the lines of structure are not parallel to the seams of stra- 

 tification, and instanced the sandstone rocks at Crichton Castle and 

 in the neighbourhood of Roslin. He expressed his conviction, that 

 Lehmann and Arduino's definition of primitive rocks should still 

 be adhered to, and regretted that the innovations in the terminology 

 of geology had increased the difficulties of the present discussion. 



Mr Lyell explained the definition of stratification he has given 

 in his Principles of Geology, and made some farther remarks on 

 the essential characters of stratification. '' 



Professor Phillips observed, that the views of geologists on th# 

 defects of the stratification and other characters of primary strati, • 

 were commonly tinctured with peculiarities depending on the li- 



