56 W. S. GRESLEY—CLAY-VEINS INTERSECTING COAL MEASURES. 
between the two layers of coal into which crept the clay forming the 
‘“horse-back.” Of course the clay-vein is the newest deposit in this 
section. 
Figures 25 and 27 are interesting examples of contemporaneous ero- 
sion and deposition in the coal period, and may serve to remind those 
who spend much time underground of numerous other instances of 
somewhat similar phenomena, and of which mental note or sketch-book 
record was made. 
Though somewhat irrelevant in this connection there are two facts per- 
taining to the clay-veins in Pennsylvania which should be noted if only 
asamatter of record: In newly developed mining territory the cutting of 
one of these veins for the first time will sometimes draw off the water in 
surface springs overhead—a circumstance which makes it evident that 
some of the clay-veins do extend to the soil above. Again, the coal next: 
above the Pittsburg bed, being full of clay-veins, confines the explosive 
gas in it, which, as many claim, issues in considerable quantities into 
the workings of the coal below as the withdrawal of the pillars proceeds. 
CoNCLUDING OBSERVATIONS 
Notwithstanding there must be a large amount of data on clay-veins 
of which the writer is ignorant, he still has reason to think that, with a 
little more attention paid to them by mine surveyors and engineers of 
mines, much valuable material might in course of time be recorded.* 
In view of the fact that none of these deposits possess commercial 
value, it may be and indeed has been urged that from the standpoint of 
practical or economical considerations there is not very much to be derived 
from such study, but the true investigator is justified in declining to give 
too much weight to such arguments and in insisting that wherever and 
whenever an opportunity to examine exposures of these veins presents 
itself, every advantage ought to be taken of it, for no two clay-veins are 
exactly alike; one always finds physical, structural, and material differ- 
ences, and all of them have their meaning, the explanation of which can 
only become clearer and more satisfactory by extended survey, scrutiny, 
and methodical collecting and arranging of facts. It is only by such 
methods that whatever material significance they may possess can be 
revealed. 
Reverting to the question of the origin of these veins, it has often 
occurred to the author when turning over the plans of metalliferous veins 
* For valuable information in this connection sincere thanks are hereby accorded to Albert M. 
Campbell, John Rutherford, A. N. Humphreys, F. T. Hogg, Selwyn Taylor, George J. Binns, and 
others, 
