486 Prof. Sedgwick on the Structure of large Mineral Masses. 



Flintshire during last summer, and of the documents kindly put in my hands 

 by Mr. Taylor, showed, in that county, a still more perfect symmetry and 

 ao-reement with the mechanical theory. The great parallel cross courses 

 range north and south, in the direction of the beds ; and the metalliferous 

 veins strike across the calcareous beds, almost mathematically at right angles 

 to the cross courses. Whatever may be the true theory of metalliferous veins 

 in some countries, there cannot be a doubt that the great joints and fissures 

 of Flintshire and Derbyshire are of a mechanical origin. 



I now terminate what I have to lay before the Society on the changes pro- 

 duced during successive periods in the structure of rocks; and so far from 

 thinking that I have exhausted the subject, I rather wish some parts of this 

 paper to be regarded as mere hints, to be followed out by better and more 

 extended observations. 



