The Ccelentera in Relation to Geological Zones. 57 



III. The Ccelentera in Relation to Geological Zones. By 

 J. G. GooDCHiLD, H.M. Geological Survey, F.G.S., F.Z.S. 



(Read 19tli March 1902.) 



One of the most remarkable facts with which the geo- 

 logist who works at the Proterozoic Eocks has to deal is the 

 very limited vertical distribution of certain species of Grap- 

 tolites. In some cases a well-marked species of this group 

 appears for the first time, perhaps in great numbers, upon 

 some definite geological horizon, and then, in the succeeding 

 stratum, entirely disappears, and its place is taken by quite 

 another form. This is especially the case with the grapto- 

 liferous mudstones of the Upper Ordovician Eocks, as well 

 as with those occurring in the lower subdivisions of the 

 Silurian strata. The fact is the more remarkable in the case 

 of these mudstones, seeing that, as a rule, they have clearly 

 been formed under conditions which, so far as one can judge 

 from the evidence, were quite uniform in character almost 

 from beginning to end. As an example of a different kind, 

 which may be compared with these, may be instanced the 

 Ammonite zones of the Jurassic Eocks. These also have a 

 very limited vertical range ; but then the several strata in 

 which the different zonal species occur are diverse in petro- 

 graphical character, and the Ammonites may therefore be 

 reasonably supposed to owe their shortness of specific exist- 

 ence to the influence of uncongenial conditions recurring 

 at short intervals of geological time. 



In marked contrast with both of these types may be cited 

 the fauna and flora of the Louver Carboniferous Eocks. The 

 strata in which these are found, especially the types of 

 Lower Carboniferous Eocks occurring in North Britain, show 

 abundant and perfectly clear evidence of repeated changes 

 of physical conditions of almost all types except those of 

 land and fresh-water conditions. Yet the organic remains 

 are virtually the same from the lowest platforms up to the 

 base of the rocks of Upper Carboniferous age. In the Lower 

 Carboniferous Eocks, indeed, one would be almost justified 

 in saying that but few zonal forms have yet been detected, 



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