SECTIONAL TRANSACTIONS.—C. 387 
BaO-content affords a delicate check on conclusions drawn concerning both the 
fact and the quantitative importance of shale-assimilation :— 
(a) In the granites, and in the granitised basic xenoliths (which range from small 
nodules to rafts) the BaO-content varies from ‘ nil’ to 0-02 per cent. In the majority 
of cases it is either ‘nil’ or ‘a trace.’ 
(6) But in the aureole shales, from various localities, the BaO-content is 0-05 and 
0:07 per cent. (for two Leusdon shales) ; 0-02 per cent. and 0:06 per cent. (for two 
Yarner shales) ; 0-03 per cent.0°11 per cent. av.; 0°07 per cent. in a serial suite of 
nine (Burrator contact). In four shale-xenoliths it is 0-03 per cent.—0-08 per cent. 
(c) The Burrator marginal granite, which has incorporated about 30 per cent. of 
shale-substance derived from the contact-shales, contains 0-05 per cent. BaO ; whereas 
the Burrator quarry-granite remote from the contact contains only ‘a trace’ of BaO. 
In the porphyritic felspars, to which shales have made substantial contributions, 
BaO amounts to 0-05 per cent.—0-08 per cent. In a cordierite-rich biotite-granite 
(Sweltor quarry), the BaO-content attains the exceptionally high figure of 0-24 per cent. 
Quantitative spectroscopic analyses afford an equally delicate check on the con- 
clusion that a large proportion of the biotite in these granites has incorporated 
magnesia, iron-oxides and titania derived from assimilated pyroxene-bearing basic 
igneous rocks. Chromium, nickel, cobalt, silver, strontium, lead, and even iridium 
are traceable to hornblendic (altered) basic xenoliths. 
; Assays for gold and silver give the following results—expressed in grains per 
ong ton :— 
Tor-granites (av. of four): 1:2 Au and 21°3 Ag; quarry-granites (av. of seven): 
2-6 Au and 46-2 Ag; an amphibole-bearing pegmatitic hybrid (av. of eleven samples) : 
64-8 Au and 117-9 Ag. 
At one period (1924-26) the writer was inclined to regard these metals as primary. 
This conclusion became increasingly doubtful as the evidence for assimilation was 
strengthened ; values are highest in the pegmatite described above. 
A check assay of biotite separated from a quarry-granite yielded 1-5 grains Au and 
108 grains Ag per long ton; still higher Ag-values are found in the biotite arising by 
reaction from amphibole in the basic xenoliths. 
Reverting to the hypothesis that the Hercynian granite-magmas originated by 
the fusion of deep-sunk fold-roots :— 
At a depth of 10 kms. the temperature is normally about 300° C. For the roots 
of a moderate fold Prof. Holmes estimates a depth-locus of about 20 kms., where the 
temperature ultimately attained would be about 1200° C.—either below or above this 
mean figure according as the original material of the fold-roots was less, or more, 
radio-active than the material forming the overlying 10 kms. 
But long before the material forming the fold-roots could be raised to a temperature 
of this order, selective (i.e. differential) fusion of sialic material would be inevitable. 
Moreover, both the estimated depth and the temperature of initial liquefaction would 
be reduced in proportion to the amount of water co-operating in the fusion. 
If the initial magma originated as a ‘ dry ’ melt, its composition would approximate 
to that of quartz—alk. felspar eutectic; on the other hand, a ‘wet’ melt would 
acquire excess silica together with alkali-silicates, basic solutes, fluxes (from 
fluoriferous micas, apatite, tourmaline, &c.) and possibly also heavy-ore metals 
present (as traces or in more significant amounts) in the rocks undergoing fusion. 
But heat adequate for the fusion of the fold-roots could be carried directly to the 
sia] by an initial uprise of basaltic magma, and this case entails the possible co-existence 
at some stage of two magmas, the one being acid and of palingenetic origin, the other 
being basic and primary ; the two could of course co-mingle. 
The source of the ores, &c., characterising the Hercynian granites presents a highly 
intricate problem, to which paleogeography and a geochemical study of sedi- 
mentaries appear to be specially applicable. 
Dr. W. R. JongEs. 
Mining geologists welcome heartily the co-operation of petrologists in the study 
of ore-deposits, particularly of those types of deposits which are so closely related to 
igneous rocks that knowledge of the formation-processes of these rocks leads to a 
better understanding of the mineralization ; the two processes are often so intimately 
related genetically as to be inseparable. 
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