part 2] THE VALENTIAlSr SERIES. 173 



underlying the Acuminatus Zone, consisting of 



' tough slightly calcareous shale weathering of a brownish drab, or ginger- 

 bread-colour (" the gingerbread band "), and affording numerous examples of 

 Climacogi'aptus scalaris (var. nonnalis) in a state of high relief.' ^ 



This horizon coincides with a marked palseontologi 3al break in the 

 succession. 



In the Lake District, J. E. Marr & H. A. Nicholson ^ describe two 

 different types of deposit at the base of the Valentian (Stockdale 

 Shales). In Skelgill Beck and other localities about 1 foot of a 

 tough impure limestone of a mottled appearance, and containing a 

 considerable quantity of iron pyrites, overlies the Ashgill Shales : 

 in it w^as found Atrypa Jlexuosa. The relation of this to the over- 

 lying Lower Birkhill Shales is nowhere seen. In Browgill 2| feet 

 of mottled, pyritous, calcareous shale occurs at the base, and is 

 considered from its close lithological resemblance to represent the 

 Atrij'pa-jiexuosa Limestone. It 3delded Diylograptus acuminatus 

 and Climacograi)tus normalis. These authors refer to a complete 

 palseontological break between the Lower Skelgill Beds and the 

 highest Ashgill Shales, although there appears to be perfect 

 conformity in dip and strike between the two formations. 



At Austwick the base of the Silurian is a fossiliferous con- 

 glomerate, which appears to mark a physical break of some 

 importance. 



At Rhayader the base of the Silurian was drawn on lithological 

 grounds below a great zone of grits (Gwastaden Grits), which are 

 followed conformably upwards by graptolitic Birkhill Shales, but 

 rest abruptly upon very dissimilar strata assigned to the Bala. In 

 all the areas so far noted the lowest graptolitic zone recorded is 

 that of ' Diplograjjfus ' acuminatus. 



In Centra] Wales I gave reasons^ for assigning the basal beds 

 of the Birkhill Grroup to a new zone, Glyptograptus 2)ersculj)tiis, 

 which underlies the Acuminatus Zone. The new zone consists of 

 blue-grey, dark-flecked or mottled, tough, compact mudstones, in 

 places slightly calcareous and in others containing a considerable 

 quantity of iron pyrites : they tend to weather snuff -brown. The 

 description of these strata recalls at once both the ' gingerbread 

 band ' of Moffat and the basal beds in the Lake District. The 

 /one-fossil occurs in profusion in a shale-band close to the base of 

 the zone. The boundary between these deposits and the Ordovician 

 has been examined at many localities over an area of about 

 200 square miles, and is always sharply defined. The overlying 

 rocks can in many places be stripped off, revealing a smooth, 

 slightly-undulating surface of the Bala rocks. There is no appear- 

 ance of a passage between the two formations, and, on the other 

 hand, no sign that the sharp demarcation between them is the 

 result of movement at the junction. 



The type of Bala in contact with the base of the Silurian is not 

 everywhere the same ; but in such unfossiliferous sediments it is 



1 Q. J. G. S. vol. xxxiv (1878) p. 252. ^ n^ia. vol. xliv (1888) p. 654. 



3 Ihid. vol. Ixv (1909) p. 482 & vol. Ixxi (1915-16) p. 350. 



