146 GEOLOGICAL INVESTIGATION. 



Moreover, they have been able, after a single glance, as 

 it were, at the two kinds of rocks, to assign the succes- 

 sive exposures of ledges, in passing over the fields, at 

 once to their right groups. 



Yet it is also beyond question a fact that the Trenton 

 rests directly upon the so-called calciferous, and is at 

 that plane so intimately in union, or amalgamated with 

 it, that it appears to grow out of it solidly, without any 

 definite seam of separation, such as is usually found be- 

 tween different strata. Thus the rock is not a whit more 

 inclined to break under a blow along a plane which would 

 separate between the Trenton and calciferous, than it is 

 in any other direction. It is not therefore uncommon to 

 find in the debris, fragments of homogeneous solidity 

 throughout, which contain portions of both formations. 

 These come from such positions of junction. Where the 

 two formations thus come together, their characteristic 

 fossils are in the closest juxtaposition. There, and there 

 only, the Ophileta compacta lies alongside of the Choe- 

 tetes compacta. But more than this : as the surface of 

 the lower rock, the calciferous, was evidently undulating 

 with ridges and hollows, at the time that the Trenton 

 fossils began to be deposited upon it, it follows that, as 

 referred to a horizontal plane, in these places Trenton 

 fossils may lie at a lower level than calciferous ones, and 

 calciferous fossils, in close proximity, may lie in the solid 

 rock at a higher level than their Trenton neighbors. 

 Thus, in a certain sense, but I think not in the full, true 

 sense of the expression, the Trenton and calciferous fos- 

 sils seem to be intermingled, for a very narrow space, at 

 the line of junction. 



Here again questions of general stratigraphical im- 

 portance are raised. If there is a true and absolute in- 

 termingling of the two fauna here, then it follows that 

 for a certain limited transitional period these two distinct 

 systems of life were synchronous. This is a point of so 



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