210 Murchisons Silurian System. 



an intermixture which affords zoological proofs that the zone in 

 question is made up of transition strata connecting the new red and 

 carboniferous systems. At the same time, (if the species be correctly 

 determined) this discovery tends to modify one of the strongly defined 

 stratigraphical characters assigned to fossil fishes by the researches of 

 M. Agassiz ; for whilst occasionally some few species in other classes of 

 the animal kingdom are known to have lived on through various 

 successive periods, each species of ichthyolite is supposed by that author 

 to be peculiar to the formation in which it is found imbedded. Now in 

 the preceding chapter it has been shown, that the formation of the 

 lower new red sandstone, having a maximum thickness of nearly one 

 thousand feet, is interpolated between the magnesian limestone and the 

 coal measures, in both of which formations, thus widely separated, we 

 now find the same species offish. It has, however, been my object to 

 show, that no violent interruption of the series of deposit of this age 

 has occurred in Shropshire or Staffordshire j and hence we might well 

 imagine, why under such conditions, animals of the same species should 

 have continued to exist during a very long period. But at Manchester, 

 the stratigraphical relations are different from those described in the 

 central counties, the red sandstone and marles, including the equi- 

 valent of the magnesian limestone before mentioned, being uncon- 

 formable to the upper coal measures and fresh water limestone : yet not- 

 withstanding this dislocation, which interrupts the perfect sequence of 

 deposits, there is still a complete transition in mineral type and organic 

 contents." 



Regarding the rocks of the upper coal measures in Shrop- 

 shire, Mr. Murchison observes they are seldom inclined 

 more than five inches in a yard, except where the coal dips 

 at high angles beneath the oldest members of the new red 

 sandstone and dolomitic conglomerate. Among the number- 

 less faults that affect the strata, the chief one runs from 

 north-north-east to south-south-west, and is a downcast of 

 seventeen yards on the dip, whereby the limestone is thrown 

 down nearly to a level with the thin coal ; as the coal ap- 

 proaches Shrewsbury it becomes thinner and more disjointed ; 

 a patch of it forms a broken and elevated trough extending 

 to Longden, where the coal rises to within a few yards of the 

 surface, resting on purple greywacke grits of the Cambrian 

 system, but is not worked. This purple greywacke advances 



