Prof. Sedgwick on the Structure of large Mineral Masses. 475 



solid ; and after that time it is not conceivable that any mere mechanical 

 force^ however violent^ should have thrown them into such contortions as we 

 often see passing- through them. Again,, the contorted laminae^ so often seen 

 in formations of argillaceous schist^ seem to be removed from all analogy with 

 known modes of crystalline action ; whereas the great parallel plates of slaty 

 cleavage (however enormous may be their scale,) are quite compatible with it. 

 After these remarks, I proceed to notice a few actual sections, and to point 

 out the conclusions we may draw from them. 



Plate XLVII. fig. 2. 



This section ranges in a direction north-west by west, and south-east by 

 east, through Mr. Penant's great slate quarry on the road to Bangor. 



1. Slate rock. 



2. Coarse greywacke sandstone. 



3. Anticlinal line in slate rock. 



r Coarse greywacke. In its prolongation it crosses the top of Moel Paban. 

 1 Sandstone. 



5. Great bed of roofing-slate. 



6. Coarse greywacke, prolonged through Fron Llwyd. 



7. Bastard slate. 



8. Very coarse greywacke, forming the crest of Lider Fawr. The dip of 

 the strata east of the anticlinal line is south-east by east, at about 45°. Enor- 

 mous excavations have been made in No. 5, but they very seldom show any 

 traces of the true beds. A few stripes are, however, to be seen, just at the 

 top of the great quarry ; and these stripes dip exactly with the beds. The 

 cleavage dips south-east by east, at about 80°, and is therefore inclined about 

 35° to the planes of stratification. This is a very good example for study, as 

 the true position of the beds, when viewed on a great scale, is very obvious. 



Plate XLVII. fig. 3. 



This section is from a small slate quarry near Harlech. 



At the top is a series of hard quartzose sandy beds («), without cleavage, and 

 dipping east by north, at 50°. Below is roofing-slate (6), which gives no indica- 

 tion of the true bedding ; but the cleavage planes dip about north-east, at GO''. 



Plate XLVII. figs. 4, and 4 a. 

 We have here represented two portions of a series of finely contorted strata 

 of hard greenish slate, from the rugged mountains on the left bank of the 



