EARLE, INTERliEDDED IRON ORE DEPOSITS 149 



of different density occur together, there seems to be a natural tendency 

 for either mechanical or chemical filling to take place, beginning along 

 the line of contact and extending a short distance into the finer-textured 

 stratum, thus protecting the latter from further penetration by the solu- 

 tions contained in the coarser layer as effectually as though the finer layer 

 had been impervious in the first place. This is well shown in various 

 specimens of cavernous consolidation (Plate XI), in whicli iri)n solutions 

 penetrated the finer sand for a short distance, filling the pore spaces so 

 completely that part of the inclosed sand remained not only uncoated 

 with iron, but even unconsolidated. 



MARINE ARTESIAN SLOPES 



Marine artesian slopes are among the common sources of artesian water 

 supply to-day. Strata of alternating sand and mud, often many times 

 repeated, are among the commonest types of in-shore deposits, and these 

 sedimentary layers as originally deposited have an initial dip. Further- 

 more, the coarser and more porous layers as they extend into deeper 

 waters tend to change their textures from coarse to fine and their contents 

 eventually from sand to mud or ooze. These conditions result iii a nat- 

 ural catch basin for penetrating water that cannot be improved upon for 

 the establishment of good artesian conditions. 



The Clinton beds comprised alternating impervious and porous layers, 

 forming the marine slopes of the epicontinental shelf of that period, a 

 condition in all respects favorable for the retention of artesian waters 

 after uplift. These runways probably were brought to an end downward 

 by a change of texture and material from porous to impervious deposits 

 seaward ; but even if such a sealing of the porous medium did not occur, 

 it is highly probable that in a comparatively short time precipitation 

 would cause a filling of the voids at the lower end of the runways and 

 accomplish the same results as a change of texture. 



INFILTRATION OF METEORIC WATER 



That conditions favorable to the easy penetration and retention of 

 ground or surface waters could exist, therefore, seems clear; and that 

 penetration of such water in these channels took place is equally well 

 indicated by field study and the microscopic examination of slides from 

 Clinton seams. This is shown bv the evident and v.'idespread corrosion 

 of the nuclei of calcite (Plates IX, XVI), and even quartz in some cases 

 (Plates XX, XXT), that form so large a part of these, deposits. If we 



