296 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



In the foregoing section there is no marked change of character 

 in the rocks at those points of the descending series where the 

 author proposes to draw a line of separation between the upper 

 schists and the lower portion of the Lower Silurian on the one 

 hand, or between the Lower Silurian and the Cambrian systems on 

 the other. 



On the eastern side of the Carnarvonshire trough, and near the 

 line of the preceding section, are the great slate- quarries which lie 

 to the north-west of Ffestiniog. As these quarries afford great 

 facilities for a minute examination of the beds, the author made a 

 detailed section of these beds, all of which he refers to the lower 

 division of the Lower Silurian formation. The series of slate beds 

 of which the description follows in detail are included in the pre- 

 ceding general account of the strata (see p. 295), in the beds num- 

 bered 13 to 7. 



Descending order of Beds in the Slate Quarries N.W. of Ffestiniog. Feet. 

 "25 Dark blue slate, not yet worked. 



24 Blue slate, too hard to work 120 



23 Hard grey chert or hornstone 1 



22 Dark blue slate of good quality 150 



21 Blue slate, too hard to work 120 



20 Hornstone 18 



19 Poor slate 3 



18 Hornstone 6 



17 Dark blue slate, of the best quality 150 



Note. All the above beds dip 30° N.W., the dip of 

 the cleavage being 55° N.W. 

 16 A whinstone dyke, which, on the face of the hill, is inter- 

 stratified with the beds of slate, but, in the quarry, cuts 

 across them. 



'15 Schists 500 



14 Slate, which is quarried 100 



13 Schists 600 



12 A-lternations of hornstone and slate, in beds from one to "I oa 



five feet J 



11 Poor slate 10 



10 Good blue slate 50 



9 Schists, about 100 



8 Whinstone, thinning away towards the top of the quarry. 



Slate, too hard to work 50 



Slate, of middling quality 100 



Good slate 60 



4 Schist, with some alternations of hornstone ; the beds 1 ^^^ 



are curved. Dip 10° to 15° N.W J ^"" 



3 Semi-crystalhne quartz-rock 100 



2 Greenstone, interstratified. 



1 Light grey slate, of indifferent quality. Dip 15° N.W. 

 Dip of cleavage 35° N.W 



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150 



Total thickness, exclusive of igneous rocks 2910 



Note. The beds 14 to 2 inclusive dip 25° N.W., the dip of the cleavage bein^ 

 45° N.W. 



In the Rhiw Brefder quarries the beds are merely intersected by iiregular 

 veins of quartz. In the Diffws quarries the rock is soft, and readily splits into 

 slate ; and the colour of the slate is lighter and of a more decided blue than in 

 the beds above. Below the beds of the Manod Mawr quarry there are some beds 

 of slate, of inferior quality, and of no economical importance. These lowest beds 

 the author refers, not to the Lower Silurian, but to the Cambrian svstem. 



