IRON CONCRETIONS OF REDBANK SANDS 



247 



the second place, the formation of long, straight, horizontal, tubular 

 cracks, or such systems of cracks as would be necessary to give the 

 observed forms to the compound concretions, would be a mechanical 

 marvel under the most favorable circumstances. They are clearly 

 not of stalactitic origin. The Redbank formation is ideally porous ; 

 where the concretions display their most typical characteristics the 

 formation does not now, at any rate, possess any structural feature 

 which may have served to guide convection currents. All the evidence 



Fig. 4. — Bluff in the Navesink Highlands, showing irregular tubes and corrugated 



sheet. 



obtainable seems to show that the material which supported the 

 growth of the concretions was transported to them by diffusion rather 

 than by convection. Transportation by diffusion here refers to 

 movement of dissolved matter in obedience only to the molecular 

 activities of the substance in solution, while transportation of the 

 same by convection implies a bodily movement of the solution in a 

 determinate direction. The distinction is important in this connec- 

 tion, since if water currents, along with other mechanical agencies, are 

 denied any part in the formation of these objects, they must be 

 regarded as an expression of a molecular tendency of the cementing 

 material. 



The evidence in favor of transportation by diffusion is clear and 



