Missing Chapters of Geological History. 17 



series ; and thus, though the number and variety of the fossils, 

 both in the Lingula flags and Treraadoc slates, is but small, 

 there is a nearly complete break both in genera and species, 

 and this accompanied by unconformability in the deposits. 



Between the Tremadoc slate and the Caradoc beds there is 

 even a more marked difference. That the former is a very 

 fragmentary, and probably a local deposit, is probable. But 

 the great change in the organic contents is certainly a remark- 

 able phenomenon, whether it results from an interruption of 

 deposit or a denudation. In any case, both slates and flags 

 are poor in fossils, except as regards Trilobites, which probably 

 mark moderate depth of water. The Lingula flags, however, 

 attain a thickness of 6000 feet in Merionethshire, and the 

 Tremadoc beds are not unimportant. Both mark the lapse of 

 considerable time, and the interval between the two must have 

 been correspondingly great. 



The lower Caradoc beds rest, apparently with conformity, on 

 the upper Tremadoc slates. They afford not only many more 

 species (the total number is thought to exceed 500), but much 

 more variety ; and though the Trilobites vary, the species gene- 

 rally are the same throughout. There is, however, another 

 decided interruption where the Pentamerus beds of Llandovery 

 (for the most part sands) overlie the Caradoc beds. With little 

 evidence of want of conformability we find great change in the 

 species. Thus, out of seventy known species, less than a fourth 

 part (sixteen species) pass upwards from the Caradoc. The 

 evidences of an important physical break at this point are very 

 decided. 



The middle Silurian series, as the Pentamerus beds of 

 Llandovery are sometimes called, consist of the lower and upper 

 Llandovery strata. The latter are chiefly conglomerates. They 

 are cut off, with some clearness of definition, both by fossils and 

 by unconformability, from the Caradoc sandstone on the one 

 hand and the Wenlock shale on the other. Great mechanical 

 disturbances had affected the lower beds before the Llandovery 

 beds succeeded them, showing a marked strati graphical 

 break, supporting the evidence derived from a comparison of 

 the fossils. 



The upper Silurian rocks form a series of some five or six 

 thousand feet of strata, including all varieties of mineral cha- 

 racter, and loaded with organisms. It may be that the lower 

 Silurians were deposited chiefly during depression, and the 

 middle beds during a period of oscillation of level. Professor 

 Ramsay distinguishes six divisions of the whole Silurian series, 

 each separated from the underlying series by breaks, and each 

 break believed to represent a lost epoch, unrepresented in our 

 area. 



VOL. vi. — no. C 



