SECTIONAL TRANSACTIONS.—C. 381 
in alluvial deposits. In the latter, valuable saline residues have arisen by the desicca- 
tion of salt-lakes and inland seas ; or, alternatively, bedded ores of iron and manganese, 
base-metal sulphides, &c., have resulted from chemical reactions set up in lacustrine 
or marine waters by the metabolism or decomposition of plants and animals. 
The surface waters with which sedimentary rocks are connected, by descending, 
may yield fillings and replacements underground, but it is probable that ore-deposits 
due to the descent of supergene waters are insignificant in comparison with those 
formed by ascending hypogene waters. By the differential leaching of rocks and 
mineral veins at outcrop, however, meteoric waters give rise to important residual 
blankets of lateritic ores and to the zones of secondary enrichment in metalliferous 
lodes, this probably being their principal ore-forming activity beneath the surface. 
Ascending magmatic waters are ore-makers ; descending meteoric waters ore-modifiers. 
Deposits of metamorphic origin are relatively unimportant. Metamorphism 
rarely initiates mineralisation; it more often changes earlier ores or protores by 
heat, pressure, &c. Most contact deposits have been due to thermal metasomatism 
by metalliferous emanations from igneous intrusions ; strictly speaking, they are not 
metamorphic but igneo-genetic. Pyrometamorphism may occasionally operate alone, 
however, as in the dehydration of fossil laterite and its conversion to emery, Dynamo- 
metamorphic deposits may be exemplified by pre-Cambrian iron and manganese ores, 
the mineral composition and physical condition of which have been economically 
bettered by recrystallisation. 
From the foregoing considerations it may be inferred that the piace of ore-formation 
in the petrogenic scheme is now realised in general. There is, notwithstanding, a 
remarkable lack of agreement as to the genesis of individual deposits or fields, and 
it is a significant fact that for practically no mining field of any consequence has 
agreement been reached as to the detailed genetic history of its mineralisation. The 
gold ores of the Rand, the silver ores of Cobalt, the nickel ores of Sudbury, the pyritic 
ores of Huelva, the copper ores of Northern Rhodesia, the lead ores of Missouri, the 
zine ores of New Jersey, the magnetic iron ores of Lapland, and the West Coast 
hematites of our own country may be cited, at random, as well-known controversial 
cases. Clearly there is still need for research and occasion for discussion. 
Mr. K. C. Dunnam. 
The Ore Deposits of the Upper South Tyne, Wear and Tees area of the Pennines 
consist of Galena and Sphalerite-bearing fissure veins which have been worked mainly 
in the Yoredale series of alternating limestones, sandstones and shales. The gangue 
minerals include fluorspar and barytes, together with quartz and carbonate-minerals. 
The assemblage of minerals is one that is known in many districts both at home and 
abroad ; a discussion of the criteria bearing upon the origin of the deposits may there- 
fore have a more than local application. 
The theories of origin fall under two heads—Supergene and Hypogene ; the rival 
possibilities include processes either of concentration from the sedimentary rocks of 
the Pennines, or of injection from a deep-seated magmatic source. 
The direction of movement of the ore-bearing solutions has been definitely estab- 
lished from observations upon inclusions in the veins; for example, clear cases are 
known in which shale has been carried upwards in the vein by the solution. The fact 
that the veins pass through soft shale beds of considerable thickness suggests, further, 
that the solutions were ascending under pressure. 
Derivation from the country-rock could only have taken place if there were an 
artesian circulation to ensure the ascension of the solutions. Comparison with the 
Joplin region of North America reveals many similarities ; for example, the monoclinal 
structure, so essential to artesian circulation; but the lack of any direct evidence 
of such a circulation, and the presence of great numbers of shale beds militate against 
hopes of proving a supergene theory. Further evidence to the contrary is provided 
by mineralogical differences (e.g., fluorspar abundance in Pennines), Observations 
in mines show that only minerals such as calcite and limonite, together with oxidation 
minerals, are actually in process of being deposited from the circulating waters, even 
in places where large quantities of pyrite would cause precipitation as sulphides. 
The evidence in favour of hypogene origin may now be reviewed. A definite 
zonal distribution of materials has been established on a large scale—a fact greatly in 
favour of magmatic origin from foci. The presence of the Whin Sill, of slightly 
earlier date than the veins, indicates the existence, in late Carboniferous and early 
