EVIDENCES OF DEFORMATION. 29 



tion in the midst of great masses of fossils indicates that a sudden incursion of 

 hydrocarbons was the cause of the death of the organisms. 



The petroleum shale aj^pears to be calcareous rather than argillaceous. Diges- 

 tion in acids causes much effervescence, and leaves a brown spongy mass, which 

 burns freely when dry. The fossil shells entombed in the shale are generally well 

 preserved and are sometimes silicified. 



Analyses of this rock show, as would be expected, a wide difference in the 

 amounts of earthy and of volatile constituents in different specimens. 



In the mines of the ShuUsburg region this "oil-rock" is regarded as marking 

 the lowest horizon of workable deposits of blende. It appears to be the floor of 

 the heaviest zinc deposits upon which the sulphides are spread out in sheets and 

 masses as if the shale were both impervious to the solutions and at the same time 

 favorable to the deposition of zinc ore in the more porous upper portions. It is 

 easy to believe that this floor of petroleum shale has acted as a reducing agent 

 upon zinc and iron solutions, while retaining them by its impervious nature. The 

 so-called " brown rock " and the '' green rock " are believed to be modified forms 

 of this oily deposit, where it is more ferruginous and clay -like and attains a greater 

 development. It marks distinctly the horizon of the top of the Trenton limestone 

 proper, the *' quarry-rock " underlying the Galena dolomite. It is thus at the 

 base of the chief mineral-bearing strata, and though there are instances of layers of 

 blende occurring below this petroleum shale, they are regarded as exceptional. 



Evidences of Deformation. 



Crevices. — The rectilinear and geometric arrangement of the mineral-bearing 

 crevices, their parallelism, right-angled intersections, and their great length as 

 compared with their breadth, are certainly evidences of some general and compre- 

 hensive cause of the fracturing. We are reminded of the experimental demonstra- 

 tions of Daubree in the torsion, to the shattering point, of the sheets of plate-glass, 

 and find in the homogeneous vitreous limestones of the Trenton, known as the 

 *' glass-rock," the bed-rock of the lead region," a material which very closely sat- 

 isfies the conditions of those experiments. 



In a region so cut up by vertical fracture planes it would be surprising if some of 

 the blocks were not displaced ; we, indeed, cannot conceive of the possibility of all 

 the blocks retahiing absolutely their original horizon plane, especially where a 

 flexed structure with its attendant strains coexisted. 



^^ Barren Bars^ — Another evidence of dislocation may be found in the generally 

 observed phenomena of walls of hard unmineralized rock, known to the miners as 

 " barren bars," which bound mining operations on one side like straight walls. 

 The formation on the one hand will be open, porous and mineral-bearing, often 

 ** picking-ground" of the miner, while on the other a different rock forms a hard, 

 unyielding, dense, barren wall, which cannot be broken without powder. These 

 bars are often narrow and separate contiguous mineral-bearing strata. Examina- 

 tion of such bars I am convinced will show vertical displacements by which strata 

 of different horizons are brought into juxtaposition horizontally. 



Relation of the Driftless Area to the Ore-deposits. 



In any attempt to explain the origin of the mineral deposits we must not omit 

 reference to the fact that the lead and the zinc region is approximately that of no 



