THE GRAPTOLITES. 369 



a few feet in thickness, being represented by thousands of 

 feet in adjoining regions, and one naturally discovers 

 forms more easily in a few feet of strata than in several 

 thousand feet where the process of search rather closely 

 approximates to that for the proverbial needle in the hay- 

 stack. The evidence which is being gathered shows more 

 strongly than ever that the thin graptolite-bearing shales, 

 which for the above reasons have come to be looked upon 

 as the deposits for graptolites /di?'- excellence, were deposited 

 slowly in waters some distance from continents, and pro- 

 bably of considerable depth. The evidence for depth 

 depends mainly on the nature of the associated organisms, 

 which are frequently dwarfed, and either blind or with 

 enormously developed eyes, whilst that for deposition at 

 a distance from land is confirmed by the ever-increasing 

 number of cases of association of graptolitic deposits with 

 others which are composed almost exclusively of tests of 

 radiolaria. The most striking" case of this has recently 

 been detected by the geological surveyors amongst the 

 rocks of the Southern uplands of Scotland (23). Messrs. 

 Peach and Home have there discovered beds with Tetra- 

 graptus of Middle Areing Age, separated from beds with 

 characteristic Glenkiln (Upper Llandeilo) graptolites by a 

 thin deposit of radiolarian chert. " We thus perceive that 

 the great mass of strata which elsewhere forms the Upper 

 Areing, and the Lower and Middle Llandeilo formations 

 are here reduced to not more than sixty or seventy feet. 

 Judged by the palaeontological evidence these thin cherts 

 appear to be a chronological equivalent of thousands of feet 

 of ordinary sediment in North Wales. They, no doubt, 

 were deposited with extreme slowness in a sea of some 

 depth, and over a part of the sea-floor which lay practically 

 outside the area of the transport and deposit of the terres- 

 trial sediment of the time." 



The graptolites are generally viewed as type-fossils of 

 the Lower Palaeozoic rocks, and this view is practically 

 correct. The earliest graptolite which has hitherto been 

 described, Dichograptus ? tenellns Linnrs., occurs in the 

 Lingula Flags of Sweden, below the shales with Dictyo- 



