PAPERS OX GEOGRAPHY AND GEOLOGY 209 



erally a smooth surface, which is apparently the re- 

 sult of solution. In size the cavities vary from ten to a 

 hundred feet in diameter and from twenty to forty feet 

 deep, and their outlines may be circular, oval, or irregu- 

 lar. They are not arranged regularly relative to one an- 

 other, but they do show a rude parallelism, controlled 

 jDrobably by joint-planes where solution would be 

 favored. Occasionally two or more- cavities may be so 

 close that their upper portions coalesce to form one large 

 pit with several separate downward projections. Along 

 the joint-planes are many smaller vertical tubular solu- 

 tion channels, also rilled with shale. 



The shale which now tills the cavities is of two general 

 types. The more abundant type is usually light green- 

 ish-gray, non-laminated, silty, and contains much crys- 

 talline iron-suliide. It also contains rounded pebble 

 limestone and chert. Frequently it is laminated, in which 

 <ome of the laminae are either fine, gray sandstone, 

 or thin, black layers of carbonized plant fragments. 

 Earely the laminae are of variegated colors of purple, 

 blue, green, and gray. This shale has a sub-conchoidal 

 fracture; it weathers rapidly to a non-plastic, non-tena- 

 cious silty, gray mud. It fills all the cavities except that 

 portion in each of a few of the larger ones which is oc- 

 cupied by the second type. The second and less common 

 type of shale found in these cavities is black, laminated, 

 carbonaceous, full of carbonized, well-preserved plant 

 fragments and brown spore-cases or seeds, and with an 

 abundance of botryoidal nodules of iron-sulfide that are 

 of all sizes up to about three inches in length. When 

 fresh, this shale has a conchoidal fracture that contin- 

 ues across several laminae. It weathers rapidly on ex- 

 posure, first dividing into sub-conchoidal, lamellar flakes 

 and then further disintegrating into a black, non-plastic 

 mud. Two or three showers with intermissions of but a 

 few days afford sufficient opportunity to reduce the fresh 

 shale to mud. 



"Where both types of shale occur in the same "pocket" 

 there is a sharp line of contact between them, and the 

 green shale contains weathered masses of the black, in- 

 dicating decisively that the green is younger than the 



