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



Towey, a little below Fanog-. Many of the beds are very quartzose, and are 

 obviously of sedimentary origin. The more schistose portions have their 

 beds defined by stripes upon the cleavage planes, and the whole contorted 

 stratification is perfectly obvious. Throughout these sections, which together 

 extend nearly a mile in length, as well as through all the neighbouring chain, 

 the cleavage planes preserve an almost geometrical parallelism, and dip to a 

 point about north-west by north. 



Plate XLVII. fig. 5. 



This represents the position of the beds, and cleavage planes, in a small 

 escarpment on the north side of the new turnpike, on the banks of Wye, a 

 few miles above Rhaiadr. Near the centre is an anticlinal line, and hard 

 quartzose beds are cracked and shattered at the point of flexure, in such a 

 way as to show that they were partly solidified when the flexure took place. 

 On the surface of some of these hard beds are casts of organic remains. The 

 upper beds on both sides are crystalline and chloritic, and have a slaty 

 cleavage, which preserves a perfect parallelism, and dips nearly north-west. 

 But these planes of cleavage affect also the more solid beds below. Though 

 of coarse mechanical texture, their component parts have been so completely 

 re -arranged, that they will only break in the direction of the slaty cleavage. 

 The length of this section is not a hundred feet, but it is extremely instructive. 



Plate XLVII. fig. 6. 



This figure represents the arched beds seen in the great slate quarry called 

 Craig y Gribbin, on the road from Llangollen to Ruthin, near the top of the 

 pass. The lower beds are coarse, and have no distinct cleavage. Among the 

 upper beds, those on the left hand of the section have their cleavage planes 

 dipping north-north-east: those on the right hand have a cleavage dipping 

 south-south-west: near the centre the cleavage is nearly perpendicular. This 

 example shows that the dip of the cleavage is sometimes effected by the dip 

 of the beds ; I believe, however, that, even in this instance, the cleavages 

 were superinduced after the beds were thrown into the arched position seen 

 in the section. 



Plate XLVJL fig. 7. 



This section represents a small portion of a calcareous slate rock, with sub- 

 ordinate beds of impure limestone, containing many corals, shells, and tri- 

 lobites. It is seen in a gorge on the west side of Foel Fawr, about two miles 

 from Llanrhaiadr in North Wales. The beds dip south-east, about 23°, but the 



