ParrIX] _ ORE DEPOSITS. 589 
termed the Paleozoic ages. It becomes further needful to discredit 
the belief that any gneiss or schist can belong to one of the later 
stages of the geological record, except doubtfully and merely locally. 
~The more thorough-going advocates of the pristine, “azoic,’ or 
“eozoic,’ date, and original chemical deposition of the so-called 
“metamorphic ” rocks, do not hesitate to take this step, and endeavour, 
by ingenious explanations, to show that the majority of geologists 
have mistaken the geological structure of the districts where these 
rocks have been supposed to be metamorphosed equivalents of what 
elsewhere are Paleozoic, Secondary, or Tertiary strata... They even 
_go so far as to assert that by mere mineral characters the crystalline 
rocks of contemporaneous periods can be identified all over the 
world. They assume that in the supposed chemical precipitation, 
the same general order has been followed everywhere over the floor 
of the ocean. Consequently a few hand specimens of the crystalline 
rocks of a country are enough in their eyes to determine the 
geological position of these formations. If geologists have discovered 
that the actual sequence of rocks is quite different, so much the 
worse for the geologists. 
In conclusion, the mode of origin of the Archean crystalline 
schists is a problem which cannot yet be satisfactorily solved. On 
the one hand it must be conceded that during the very ancient 
periods in which they were deposited, the composition of the 
waters of the ocean may have been very unlike what it afterwards 
became, and there may have been chemical precipitates on the sea- 
floor, such as could not have been formed in later and cooler times 
when life had already appeared on the earth. On the other hand, 
the striking resemblance in structure and composition between the 
crystalline schists and rocks which can be proved to be the meta- 
morphosed equivalents of ordinary sedimentary strata renders it 
highly probable that these ancient schists, whatever the circumstances 
ot their original formation, have undergone plication, crumpling, and 
metamorphism analogous to that of younger formations in areas of 
regional metamorphism.’ 
Part IX.—OreE Deposits. 
Metallic ores and other minerals that are extracted for their 
economic value occur in certain well-marked forms which have been 
1 See Sterry Hunt’s Chemical Essays, p. 382 sq. 
2 Besides the works already cited on Metamorphism the student may consult the 
following: Delessc, Mem. Savans KHitrangers, xvii. Paris, 1862, pp. 127-222; Ann. des 
Mines, xii. (1857); xiii. (1858); Daubrée, Ann. des Mines, Sth series, xvi. p. 155; 
Bischof, “ Chemical Geology,” chap. xlviii.; J. Roth, Abhandlungen Akad. Berlin, 187); 
1880; Giimbel, “ Ostbayerische Grenzgebirge,” 1868; H. Credner, Zeitsch. Gesammt. 
Naturwiss. xxxii. (1868), p. 353; N. Jahrb. 1870, p. 970. 
3 The following works on ores and mining way be consulted: B. von Cotta, “ Die 
Lehre von Erzlagerstatten,’ 1859-61; A. von Groddeck, “ Die Lehre von den Lager- 
statten der Erze,’ 1879; W. Forster’s “Treatise on a Section of the Strata from New- 
eastle-on-Tyne to Cross Fell;” W. Wallace’s “ Laws which regulate the deposition of 
