Ch. XXVII.] LLANDEILO FLAGS. 567 



North, and South "Wales, through a vast depth of shaly beds, inter- 

 stratified with trappean formations of contemporaneous origin ; these 

 consist of tuffs and lavas, the tuffs being formed of such materials as 

 are ejected from craters and deposited immediately on the bed of the 

 ocean, or washed into it from the land. According to Professor 

 Ramsay, their thickness is about 3300 feet in North Wales, including 

 those of the Lower Llandeilo. The lavas are felspathic, and of por- 

 phyritic structure, and, according to the same authority, of an aggre- 

 gate thickness of 2500 feet. 



Lower Llandeilo Formation, Murchison ; Arenig, Sedgwick. — Next 

 in the descending order are the shales and sandstones in which the 

 quartzose rocks called Stiper-stones in Shropshire occur. When the 

 term "Silurian" was given by Sir R. Murchison, in 1835, to the 

 whole series, he considered the Stiper-stones as the base of the Silu- 

 rian system, but no fossil fauna had then been obtained, such as could 

 alone enable the geologist to draw a definite line between this mem- 

 ber of the series and the Llandeilo flags above, or a vast thickness 

 of rock below which was seen to form the Longmynd hills, and was 

 called " unfossiliferous graywacke." Professor Sedgwick had de- 

 scribed strata now ascertained to be of the same age as largely de- 

 veloped in the Arenig mountain in Merionethshire, in 1843, and the 

 Skiddaw slates, studied by the same author, were of corresponding 

 date, though the number of fossils was, in both cases, too few for the 

 determination of their true chronological relations. The subsequent 

 researches of MM. Sedgwick and Harkness in Cumberland, and of 

 Sir R. I. Murchison and the Government surveyors in Shropshire, 

 have increased the species to more than sixty. These have been 

 examined by Mr. Salter, and 



shown in the last edition of Fi s- 656 - 



"Siluria" (p. 52, 1859) to *±^S&&J^, 



be quite distinct from the s^^^^^^^^^^ 

 fossils of the overlying Llan- 



, ., „ A ,i t Didymograpsus geminus, Hisinger, sp. 



deilo nags. Among these the Sweden. 



Lingula plumbea, JEglina 



binodosa, Ogygia Selwynii, and Didymograpsus geminus (fig. 656), 



and D. hirundo, are characteristic. 



In reference to the classification of the Silurian rocks, two questions 

 have been raised ; first, whether the Lower Silurian, comprising the 

 Caradoc and Llandeilo beds already described, should be separated 

 from the Upper Silurian under some new title, such as Cambro- 

 Silurian ; and secondly, whether, if we reject this, the Arenig or Stiper- 

 stones group (Lower Llandeilo of Murchison) should be regarded as 

 the base of the Lower Silurian or as the top of a distinct and older 

 series. In reference to the first question Sir R. Murchison, in his im- 

 portant work above cited,* has given a list of no less than fifty or 



* Siluria, p. 485. 



