THIRD REPORT OF THE DIRECTOR I906 35 



cavity is enlarged beneath thin or loosely coherent beds, these, 

 deprived of support, fall into it and unless beds sufficiently firm 

 to maintain their position lie above, the cavern will speedily be 

 reduced to a ruin. Such '' dead " caverns have a notable effect on 

 topography, produce small sink holes or large rock basins with or 

 without outlet and inlet. Though dead as caverns these areas may 

 still be active fields of solution and erosion. Some instructive 

 instances of such topographic forms, Karsteii, resulting from the 

 falling in of the caverns are to be seen on the Helderberg plateau. 



Ideal conditions for the formation of large caves in thin-bedded 

 limestone are furnished by the succession of hard and soft strata 

 composing the Manlius zone. The Manlius is the cavern formation 

 proper though passages in the Rondout and Cobleskill strata have 

 been worked out where favorable conditions exist. The massive 

 and resistant Coeymans limestone which yields slowly to erosive 

 forces serves as a solid roof and a protection for the weaker rock 

 beneath. The openings in the Coeymans limestone are all of the 

 nature of deep vertical shafts worn along joint planes, and occa- 

 sionally extending through the Manlius beneath. These shafts, 

 locally known as " rock holes " occur at the bottom of broad 

 trenches cut in the overlying New Scotland shale or upon high 

 land where the shale has been nearly or quite removed. In the 

 former case they receive the drainage of a considerable area and 

 may be assumed to be in process of development ; in the latter little 

 or no water reaches them, their development together with that 

 of the passages leading from them is at an end and they are of 

 importance only as indicating former lines of surface drainage. 



In this zone the main channel of every cavern investigated lies at 

 the base of the Manlius, showing that the first water to penetrate 

 the rock met with little opposition in making its way through the 

 beds above that level. Some of the side passages for a greater or 

 less distance from the foot of the shafts in which they originate, 

 have been excavated above resistant beds higher in the series and 

 in Howe's cave certain parts of the axial cavity have been formed 

 by the deepening of channels first excavated in the upper strata 

 (notably the "Bridal Chamber" and "Washington Hall") and 

 other parts by the undermining of such channels (as the section 

 filled by the " Rocky Mountains "). 



• Little could be learned of the caverns in the Becraft limestone; 

 three were found but only one was accessible. It appears from 

 the evidence obtainable that solution begins in the lowest beds, 



