POTSDAM SANDSTONE. 263 



TKILOBITES. 



Conocephalites diadematus, H. 

 Conocephalites minor, Slium. 

 Conocephalites Gibbsi, n. sp. 1 

 Dicellocephalus Misa, H. 



This is probably very far from being a fair representation of the 

 actual fauna, but the meagerness of the outcrops in this region, and 

 the fact that the rock is very rarely quarried, make it practically 

 impossible to secure a full collection. 



Method of Deposit. That this sandstone was deposited beneath 

 the ocean is shown by the remains of marine life found in it. That 

 the water was comparatively shallow is indicated by the ripple marks 

 and beach structure that abound in the formation. The rounded and 

 yet irregular character of the grains of sand that constitute the chief 

 element of the rock leaves no doubt that they were originally small, 

 angular grains of quartz that have been worn by friction to their 

 present form. The fact that this formation lies upon the surface of 

 the Archaean rocks, which abound in quartz in the form of irregular 

 grains and crystals, and in mica, feldspar and other minerals found 

 in the sandstone, that these rocks have been decomposed and eroded 

 to an enormous extent, and that they were, at the time this deposi- 

 tion was in progress, exposed to the action of the waves and atmo- 

 spheric influences, make it certain that the sandstone was derived 

 from these older rocks, and that this was accomplished by the same 

 process of wear, decay, and redeposit that is in action at the present 

 time, producing similar accumulations of sand, that may in time be- 

 come hardened to rock. The clayey material was doubtless derived 

 in the same way from the feldspar and other aluminous ingredients 

 of the same granitic rocks, but the calcareous portion was doubtless 

 chiefly formed through the agency of marine life. 



A microscopic examination of the grains of sand is entirely fatal 

 to the view still occasionally advanced, that they were produced by 

 crystalization from solution, as they neither have in general the crys- 

 talline nor concretionary form, nor one that would naturally be de- 

 rived from either of these by friction, if indeed friction were suppos- 

 able under that theory. 



Extent. It has heretofore been remarked that the Potsdam sand- 



1 The names of new species given in this volume are from the manuscript of Prof. 

 Whitfiekl, which will be published at an early day. They are here introduced for the 

 obvious value they will give the report when the descriptions shall be published, and 

 with no reference to any claim to priority of publication. 



