Feather stonhaugK^s Geological Report. 33 



schists, passing occasionally into micaceous and quartzose 

 slates, and contains subordinate masses of white granular lime- 

 stone and serpentine rock. In other localities, masses of dark 

 glossy clay slate, devoid of calcareous matter, are found, and 

 passing into the inferior primordial rocks. This lower Cam- 

 brian formation appears to contain no organic remains, but 

 lead and copper are found in it. 



The middle Cambrian of Caernarvon and Merionethshires 

 contains great quantities of fine roofing slate, often imbedded 

 in slate rocks of a coarser quality ; these alternate with and 

 apparently pass into irregularly interstratified masses of por- 

 phyry. At the top of Snowdon a few organic remains are 

 found, resembling others observed at Tintagel, in Cornwall. 

 Some of these slates are highly calcareous, but no continuous 

 beds of limestone have been observed amongst them. 



The upper Cambrian^ as observed in South Wales, is con- 

 nected with the superincumbent formation of Llandeilo flags. 

 Beds of limestone and calcareous slates occur, together with 

 organic remains. The roofing slate of this division splits in a 

 direction transverse to the stratification, which is a property 

 perhaps of all hard slates. The cleavage of this mineral is 

 considered to be the effect of crystallization. 



The ' Llandeilo flags. The Silurian system, of which this 

 is the lowest division, abounds much more in calcareous mat- 

 ter than the rocks just enumerated, and organic remains are 

 more common. These Llandeilo flags rest upon the Longmynd 

 various-colored sandstones, conglomerates, schists, and coarse 

 slates, and are dark-colored calcareous rocks, naturally sepa- 

 rating into flags. They contain, also, some sandstone and 

 schists. The asaphus buchii, the agnostus, Brongn, and some 

 other trilobites, diff"ering from those of the superior divisions, 

 which will be described in Mr. Murchison's work on the 

 Silurian system, now in the press, aie found here. 

 3 



