578 LOWER CAMBRIAN— LONGMYND GROUP. [Ch. XXV1L 



also a pliyllopod crustacean (fig. 661), and several genera of Brachio- 

 poda, -with a rare Cystidean and a sponge, were obtained. In all, 

 about forty or forty-five species are already described by Mr. Salter, 

 and other forms are still in bis bands for investigation. 



In Merionethshire, says Prof. Ramsay, the Lingula flags are from 

 5000 to 6000 feet thick ; in Carnarvonshire, near Llanberis, only 

 about 2000 feet, having, in the space of about 11 miles, lost 4000 

 feet of their thickness. In Anglesea and on the Menai Straits, the 

 Llandeilo and Bala Beds lie directly on (Lower) Cambrian strata, 

 both the Lingula flags and Tremadoc slates being absent.* 



LOWER CAMBRIAN. 



[Longmynd Group.) 



Harlech Grits. — Older than the Lingula flags are stratified forma- 

 tions of great thickness, but which have as yet proved very barren 

 of organic remains, and have been variously called by Prof. Sedg- 

 wick the Longmynd and Bangor group, comprising, first, the Bar- 

 mouth and Harlech sandstones, and secondly, the Llanberis slates. 

 The sandstones of this period attain in the Longmynd Hills in 

 Shropshire a thickness of no less than 6000 feet, without any inter- 

 position of volcanic matter. In some places in Merionethshire they 

 are still thicker. The labors of Mr. Salter in Shropshire and those 

 of the late Dr. Kinahan in Wicklow have brought to light at least 

 five species of Annelides in these rocks, two of which have been 

 named Arenicolites sparsus and A. didymus. They occur in count- 

 less myriads through a mile of thickness in the Longmynd, where 

 also an obscure crustacean form has been discovered and named 

 Palceopyge Ramsay i. The sands of this formation are often rippled, 

 and were evidently left dry at low tides, so that the surface was dried 

 by the sun and made to shrink and present sun-cracks. There are 

 also distinct impressions of rain-drops, like those figured at p. 490, 

 on many surfaces.f 



Llanberis Slates. — The slates of Llanberis and Penrhyn in Car- 

 narvonshire, with their associated sandy strata, attain a great thick- 

 ness, sometimes about 3000 feet. They are perhaps not more ancient 

 than the Harlech and Barmouth beds last mentioned, for they may 

 represent the deposits of fine mud thrown down in the same sea, on 

 the borders of which the sands above mentioned were accumulating. 

 In some of these slaty rocks in Ireland, immediately opposite Angle- 

 sea and Carnarvon, two species of zoophytes have been found, to 

 which the late Prof. E. Forbes gave the name of Oldhamia. They 



* Anniversary Address, Geol. Quart. Journ., vol. xix. p. 39, 1863. 

 f Salter, Quart. Geol. Journ., vol. xiii., 1857. 



