Review of 'Recent Geological Literature. 195 
iferous or Blue Limestone. It was in this limestone that the greater 
part of the ores were deposited, and the original deposition must have 
taken place after the intrusion of the porphyry and hefore the uplift of the 
range. 
In the uplift of the range both eruptive sheets and sedimentary beds, 
with the included ore deposits, were plicated and faulted, and by sub- 
sequent erosion an immense tliickness of rocks has been carried 
away laying bare the very lowest rocks in the conformable series; the 
outcrops are, however, frequently buried beneath what is locally called 
" wash," a detrital formation of glacial origin. In the Leadville region, 
owing to the reduplication caused by faulting, a series of outcrops of 
easterly dipping beds of the Blue Limestone are exposed beneath the 
wash, of which all are metalliferous and a considerable proportion carry 
pay ore. 
The principal ore deposits of Leadville occur, as above indicated, in 
the Blue Limestone and at or near its contact with the overlying bodies 
of porphyry. The ores- consist mainly of carbonate of lead, chloride 
of silver and argentiferous galena, in a gangue of silica and clay, with 
oxides of iron and manganese and some barite. These materials are 
mainly of secondary origin, and result from the alteration by surface 
waters of metallic sulphides. 
The study of these deposits has shown: i, that they were originally 
deposited as sulphides, and probably as a mixture, in varying proportions, 
of galena, pyrite, and blende ; 2, that they were deposited from aqueous so- 
lutions: 3, that the process of deposition was a metasomatic interchange 
between the materials brought in by the solutions and those forming the 
country rocks, consequently that they do not fill pre-existing canities; 
4, that the ore currents from which they were deposited did not come di- 
rectly from below, but were more probably descending currents; and 5, 
that these currents probably derived the material of which the ore de- 
posits are foriTied mainly from the porphyry bo('.i;s which occur at hori- 
zons above the Blue Limestone. 
Inasmuch as the ore currents did not ctnne directly from below, it is 
not advisable to search for ore behMV the Blue Limestone horizon. 
This horizon, however, should hi: thoroughly prospected, and the maps 
and sections show its probable position in the as yet unexplored areas; 
the explorations, moreover, should not be confined to the upper surface of 
this limestone, but carried into its mass wherever there are indications of 
ore, and especially along the contact of tranvcrse bodies of gray porphyry. 
The probabilities are that very considerable bodies of ore remain as yet 
undiscovered, and the most promi.'-ing areas for pros]-)ecling are indica- 
ted. It is also probable that as the distance from the surface increases 
the ores will be found less altered, and that tlieji will therefore be less 
easily reduced by the smelting processes now employed. 
Note on fossil tvood and other flant rcinains^from the Cretaceous ami I.Mr- 
.'tjnie formations of the xvrslern territories of Canada. By Sir Wilham 
