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composition of the associated blende, and perhaps to the formation of 

 hydrocarbonate of zinc — a mineral which heretofore was known to occur 

 only in small quantities, but which has been formed in very large quan- 

 tities indeed in this district. 



The hydrocarbonate of zinc is chiefly found in the limestone underly- 

 ing the dolomite. The most remarkable deposit of it is that which occurs 

 at a mine called Dolores, in the valley of Udias. As this deposit is interest- 

 ing from several points of view, a description of the circumstances under 

 which it occurs wiU, while offering several peculiar features, explain 

 the general conditions under which all the similar deposits are found. 

 The northern escarpment of this valley presents the following ascending 

 succession of rocks : — 



1. E,ed sandstone and clay beds, with accompanying gypsum. 



2. Yery shelly limestone. 



3. Sandstone and beds of clay. 



4. Irregular beds of limestone and dolomite, — the under bed pro- 

 ducing a good hydraulic lime. 



5. Shelly limestone, containing abundance of oyster- shells. 



6. Dolomite. 



7. Tertiary limestone. 



8. Tertiary greenish sandstone. 



There appears to be a fault in the direction of the axis of the valley 

 through which a stream runs, which has produced a downthrow on the 

 south, equal to the thickness of the upper beds of ]S"o. 1, and the whole 

 thickness of !N"os. 2 and 3 ; so that the bed of limestone producing hy- 

 draulic cement has been brought in contact with red sandstone of the 

 northern side. 



The dolomite contains yellowish-red Smithsonite, while the subjacent 

 shelly limestone contains the hydrocarbonate associated with silicate of 

 zinc. The ore is irregularly dispersed in the spaces between the planes 

 of stratification, and in the vertical joints. The beds of limestone have 

 only a very feeble dip, — not more than from 10° to 15°. The joints are 

 very regular, and nearly vertical to the plane of bedding ; so that each 

 bed is not unlike a great pavement, in which a block gives way, if not 

 directly sustained by the subjacent bed ; hence, caverns are easily formed 

 in such a rock. A shaft was sunk into this rock near its junction with 

 the dolomite, and a depth of about lO'^to 12™ had been attained, when 

 the workmen came upon an opening into such a cavern ; and on descend- 

 ing into it, they discovered some fossil bones upon the floor, among which 

 were recognised some teeth of an elephant in an excellent state of pre- 

 servation, and some broken antlers. This interesting circumstance led 

 one of us (Mr. O'Reilly), in company with M. Javot, the head engineer 

 of the mines, to visit the cavern. On descending into it, the visitors 

 were struck by the appearance of the roof and floor ; from the former 

 descended stalactites of various sizes, and of most fantastic forms. 



R. I. A. PBGC. VOL. VIII. 



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