Geologica I Society . 321 



and endeavours to confirm, by new arguments, the conclusion he 

 drew from the phenomena, viz., that all the complicated concretions 

 in the formation of the magnesian limestone, have been produced since 

 the original deposition of the beds*. 



4. Rocks of Globular Structure subordinate to the Old Slate Forma- 

 tions of North Wales. — He describes these rocks in considerable de- 

 tail, and divides them into two classes, both of which he is disposed 

 to arrange among stratified rocks altered by igneous action ; and he 

 remarks, that whether this opinion be true or false, the phenomena 

 illustrate a great principle in the segregation of mineral masses. 



5. Nodular Ironstone, fyc. in Beds of Shale. — These, again, are pos- 

 terior to the deposition of the beds j for it is shown (especially by 

 some examples derived from Yorkshire) that the laminations of de- 

 posit may be traced through the nodules themselves. In this case the 

 segregation of the nodule has often been occasioned by the presence 

 of an extraneous body. Other examples are given of a similar che- 

 mical segregation from a similar cause ', and the section concludes 

 with an enumeration of some appearances exhibited in the mineral 

 structure of petrifactions. 



§ 3. Slaty Structure, Cleavage, 8$c. 



The subjects introduced in this section of the paper are described 

 in considerable detail. The author first compares the structure of the 

 great Cumbrian zone, of green slate and porphyry, with the structure 

 of the principal chain of North Wales, and shows their perfect ana- 

 logy. In one respect, however, the two regions are remarkably con- 

 trasted. The Cumbrian system has few contortions or undulations, 

 probably in consequence of the great abundance of alternating beds 

 of porphyry; whereas a transverse section through the Welsh chain, 

 exhibits a continued series of longitudinal anticlinal and synclinal 

 lines. He also compares the structure of an upper slaty series in 

 Westmoreland and Lancashire, with a corresponding upper series in 

 North and South Wales, pointing out the circumstances in which 

 they agree and in which they differ. 



In all these regions occur many beds with a slaty cleavage, which 

 is entirely distinguished from a jointed structure by its indefinite sub- 

 division, and it is never found to coincide with the true plane of strati- 

 fication. These planes of stratification and cleavage sometimes dip 

 to the same point, and sometimes to opposite points of the compass ; 

 they are stated to be inclined to each other, sometimes at an angle 

 less than 10°; on the average at an angle of 30° or 40°, and in no 

 instance at 90°. Where the slaty structure is well developed, the 

 strike of the cleavage planes coincides nearly with the strike of the 

 beds j and this important rule holds true in countries where the beds 

 themselves are thrown into a series of anticlinal and synclinal planes. 

 The author adds, that there are regions in North and South Wales, 

 thirty miles in extent, and many miles in breadth, where the cleavage 

 planes (notwithstanding the numberless contortions of the beds) pre- 

 serve an undeviating direction and dip. He states that in many large 



* See Geological Transactions, Second Series, vol. iii. p. 94, et siq. 

 Third Series. Vol. 7. No. 40. Oct. 1835. 2 T 



