Theory of Formation of Sedimentary Deposits. 131 



the axis, in the township of Landsdown and elsewhere 

 where the Potsdam types of fossils are found associated 

 with types characteristic of the Calciferous, the portion of 

 the bed in which they occur may justifiably be considered 

 as a transitional phase in a horizontal as well as a 

 vertical direction towards the latter formation. To 

 make the proposed tentative interpretation clearer, Fig. 4 

 diagram i tally represents a portion of an ellipsodial 

 basin in which deposition, accompanied by slow depres- 

 sion of the oldland surface, has been going on for 

 some time. The earlier shore lines are shown in cross- 



Figuke 4. 



section at the points where the younger strata overlap 

 them to rest upon the crystalline area. Subsequently in 

 the history of the basin the land may rise somewhat, and 

 may be even slightly tilted, and the sea withdraws. 

 During the progress of degradation the greater portion of 

 the sediments are removed ; but suppose small remnants 

 of these are still to be found, it can readily be seen that 

 a small portion of the shales or limestones of, say, bed 

 number five might be left along the front of the section 

 of the basin, while towards the back a portion of the 

 sandstone of bed number three remains in a protected 

 hollow among the crystallines. It would be exceedingly 

 difficult, if not impossible, to correlate the two, particu- 

 larly if they were some miles apart. But since the sand- 

 stones beneath the remnant of bed number five are 

 identical in composition with the remnant of bed number 



