HARLECH AND LONGMYND SERIES. 53 



ferruginea) was obtained by him from the lower beds of this group 

 as early as 1867.^ 



These indicate the marine origin of the strata in which they 

 occur. Sir A. C. Ramsay has suggested that some of the red or 

 purple beds, which are unfossiliferous, may have been deposited in 

 inland waters, or in lacustrine areas subject to occasional influxes 

 of the sea.' 



Harlech Grits. 



A large tract of ground between Barmouth and Harlech, extend- 

 ing eastwards to Craig-y-Penmaen in Merionethshire, is composed 

 of greenish grits (the Harlech Grits and Barmouth Sandstones), 

 interstratified here and there with green and purple slates. The 

 series is stated by Sir A. C. Ramsay to be more than 6000 feet in 

 thickness. The beds are pierced by dykes of igneous rock. 



Evidence of sun-cracks and rain-drops has been detected on the 

 surfaces of some of the beds. The only organic remains are 

 Annelide tracks and borings. 



The junction of the Harlech Grits with the Menevian strata is 

 well seen at Aber-rhamffroch, also in the Waterfall Valley, near 

 Maentwrog, and in the Mawddach Valley, Dolgelly : the beds are 

 conformable. The slates have been worked for economic purposes 

 in places. 



Llanberis Grits and Slates. 



This series, named by Sedgwick, includes the famous slates 

 (Caernarvon or Bangor slates) of Penrhyn and Llanberis : it com- 

 prises about 1300 feet of purple and green grits, resting on blue, 

 purple, and green slates ; while below are bands of slate, grit, and 

 conglomerate. Altogether the series attains a thickness of about 

 3000 feet. The best sections are in the Passes of Llanberis and 

 Nant Francon.^ The beds contain some igneous dykes, while the 

 cleavage is very intense and distinct from the bedding. Sedgwick 

 observed that in the fine quarries of Nant Francon and Llanberis, 

 the cleavage planes strike exactly with the beds, but are inclined at 

 a greater angle.* 



North Wales is the chief slate-producing country in the world ; 

 some beds were worked as early as the twelfth century. 



At Penrhyn the slaty series comprises : — 



Green slates. 



Purple slates. 



Blue and purplish-blue slates. 



Purple and red slates. 



1 Davidson, G. Mag. 1868, p. 306. 

 ^ Q. J. xxvii. 197, 241. 



^ Ramsay, Geology of North Wales, edit. 2, p. 173. 



* Proc. G. S. iv. 215. See also diagram of Mr. Assheton Smith's quarries, 

 Llanberis, Q. J. iii. 138. 



