158 Conglomerates in Gneissic Terranes — ■A.Winchell. 
with fissure walls existing in it after solidification. The late 
Prof. Irving however, may be regarded as dissipating finally* 
any such allusions in reference to the cupriferous conglomer¬ 
ates^ since, as geologists generally have discovered, “the 
pebbles are only in very subordinate quantity of ‘trap 1 or 
amygdaloid, being almost wholly of some sort of acid eruptive 
rock, i. e . felsite, quartziferous porphyry, quartzless porphyry, 
granitic porphyry, augite syenite or granite. The fundamental 
difference between such pebbles and the associated basic, mass¬ 
ive rocks is alone enough to overthrow the theory, even were 
there not other sufficient arguments against it. Further, the 
pebbles are just as plainly water-worn as those of any other 
conglomerates, though they may have, in some cases, had the 
polish removed by surface alteration/ 1 
The evidence for the igneous origin of the Saganaga pebbles 
is incomparably less than that for the Kewenaw pebbles. An 
attentive consideration of the case confirms this conclusion. 
The conglomerate described on Wonder island is not one con¬ 
sisting originally of a mass of pebbles over which a fluid mag¬ 
ma has been poured at some date perhaps long subsequent to* 
the formation of the pebble deposits. I have seen a pile of 
angular fragments over which fluid gabbro had been poured,, 
which fioTved into the interstises and filled them. But the pre¬ 
existing fragments were self-supported—they lay in direct con¬ 
tact with each other. On Wonder island the pebbles are not in 
contact; they could not have lain where they are before the 
gneissic magma existed. The gneissic magma was present, and 
it was this which supported the pebbles and prevented their 
contact. The gneissic magma was contemporaneous with the 
pebbles. But its condition was not that of molten fluidity, for 
so vast a molten mass would have fused the comparatively few 
pebbles immersed in it—still more the single pebbles which we 
find so widely distributed. It must however, have been 
sufficiently fluid or plastic for extraneous bodies to be moved in 
it. But a molten sea would have destroyed the pebbles and 
obliterated all traces of them. The plasticity therefore, was 
low-temperature plasticity —igneo-aqueous plasticity. 
We cannot, to avoid such a conclusion, seek to propound the 
tlrving. The Copper-bearing Bocks of Lake Superior , 1883, U. S. GeoL 
Surv., Mon. v. pp. 9, 31-2. 
