SHARPE ON NORTH WALES. 311 



an angle less than 25°, and never less than 20° ; and that whenever 

 the angle is less than 20°, the slate is of inferior quality. An in- 

 crease in the angle at which the planes meet has no injurious effect ; 

 for in many instances when the slate is of the best quality, the angle 

 of intersection is 45° and upwards. 



The Igneous Rocks of North Wales, arranged according to their age. 



The oldest igneous rocks in North Wales are the greenstones of the 

 hills north of Barmouth, which are truly interstratified and contem- 

 poraneous with the Cambrian schists of these hills. As these schis- 

 tose beds could not have been deposited in their present inclined 

 position, they must have been elevated since they were deposited. 



The next igneous rocks, in point of age, are the parallel dikes of 

 greenstone, striking north-east and N.N.E., which intersect Carnar- 

 vonshire. The whole of these had assumed nearly their present po- 

 sition before the Wenlock rocks were deposited, and also before the 

 Cambrian and Lower Silurian slates were affected by cleavage : for 

 these dikes are never continued into the Wenlock series ; nor, if any 

 disturbance in the rocks which the dikes traverse can be traced to 

 the proximity of these dikes, is the Wenlock series ever affected by 

 that disturbance ; and in the beds which are broken up and tossed 

 about by the eruption of these greenstones, the cleavage planes pre- 

 serve their parallelism, as was pointed out in describing the purple 

 slates to the west of Snowdon. Some of the dikes of this period 

 have thrown the strata which they traverse into great confusion ; 

 and this is especially the case in the centre and to the west of the 

 Snowdon chain. Those on the east of that chain have for the most 

 part the appearance of interstratification with the bedded rocks. It 

 may be doubted, however, whether they are ever really contempo- 

 raneous with the beds. On the surface they seem to dip regularly 

 between the beds of schist ; but when laid open in the quarries they 

 are found to swell out, and to thin off irregularly, or even to cut 

 through the strata. These dikes are not all of the same age, but 

 they are all of earlier date than the Wenlock rocks*. 



Next in order of time come the eruptive porphyries and felspathic 

 rocks of Caderldris, Arran Mowddy, the Arenigs, and the igneous 

 plateau of the Ceiriog described by Mr. Bowman. The eruption of 

 these masses, while elevating the rocks in their neighbourhood (in- 

 cluding those of the Wenlock series), has broken up the regularity 

 of the planes of cleavage. But the Ludlow rocks lie undisturbed in 

 the hollows between the elevated masses. This eruption must there- 

 fore have taken place after the deposition of the Wenlock series, and 

 prior to that of the Ludlow rocks ; and to the disturbing forces at 

 work in this interval of time may be attributed the want of confor- 

 mity between the Wenlock and the Ludlow formations, which is 



* The dikes laid down on the accompanying map are such as the author actu- 

 ally observed, but it is probable that there are many more which are still to be 

 discovered, 



