22 Messrs. Foster and Whitney on the 
so disturbed and forced asunder by igneous protrusions, and so 
metamorphosed by direct and transmitted heat, that it is impos- 
sible to trace their continuity except over limited areas. If we 
could unfold these beds, and stretch them out in a nearly hori- 
zontal position, as when first deposited, they would require a far 
greater space than they now occupy. he causes by which 
these foldings have been effected, will be discussed in a subse- 
quent chapter. 
We have thus described the range, extent and mineral pecu- 
liarities of a series of rocks, detrital in their origin, interposed 
between the granite and the base of the Silurian system. 
Throughout their whole extent, they are more or less metamor- 
phosed, presenting a series of gradations, represented at one 
extreme by crystalline gneiss and compact hornblende, and at 
the other by bedded limestone and ripple-marked quartz. To 
the presence of granite and trappean rocks this transformation 1s, 
in a great degree, to be attributed. Much of the compact horn- 
structure as it recedes from the lines of igrieous outburst, we 
cannot but regard it as the more highly metamorphosed portions 
of the dark-green chlorite slates. ‘This compact hornblende is 
not to be confounded with those lenticular-shaped masses 
observed in the slates, which, we doubt not, are trappean in their 
nature. 
We have seen that those igneous causes which produced nu- 
merous axes of elevation, and folded the strata into a series 0 
flexures, had ceased to operate before the deposition of the 
Silurian groups, since they are found to repose in a nearly hori- 
zontal position upon the upturned edges of the slates, or to occupy 
the sinuosities in the granite, nowhere exhibiting traces of meta- 
morphism or derangement of the strata. We do not now allude 
to the renewal of those igneous causes as manifested on Kewee- 
naw Point and Isle Royale during the Silurian epoch, producing 
a class of igneous products widely different from those associated 
with the rocks of the azoic system. In a former report (Part I.) 
we have described the igneous rocks of the Silurian epoch as ap- 
pearing under a variety of aspects, such as crystalline greenstone, 
porphyry, granular trap, and a highly cellular amygdaloid, differing 
little from modern lava, except that the cells are filled with various 
zeolitic minerals. 
From the local details above given, it will be seen that the 
igneous rocks of the azoic period, though crystalline, compact, 
and occasionally porphyritic in their texture, are never amygda- 
loidal; and hence we infer that they were produced under 
widely different conditions. The latter may have been con- 
ot ae etme ones 
a 
eee pre ee Rt pancetta 
etal aie dia 
PERRIS Bac re nd 
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