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638 STRATIGRAPHICAL GEOLOGY. [Boox VI. 
Though no general order of succession has been observed among 
Archeean rocks, there is usually a difference between the texture of 
the lower and upper parts. The former are commonly coarser and 
more granitoid. They consist mainly of gneiss, with bands and 
veins of granite. The upper portions, less coarsely crystalline, are 
composed of mica-schists, tale-schists, chlorite-schists, and clay-slates, 
among which veins of granite and other crystalline massive rocks 
are less frequent. In Central Hurope there appears to bea gradation 
from the lowest up into the latest portion of the thick Archean 
series. In Canada, however, a marked line of unconformability 
exists between the gneisses (Laurentian) and the overlying slates, 
conglomerates, quartzites, and limestones (Huronian). Logan even 
traced an uncontormability between the lower and upper part of the 
gneiss. The occurrence of occasional bands of coarse conglomerate 
among the Archean rocks in different countries, especially in Canada, 
where they occur among the limestones and schists, points to 
elevation of land, and littoral erosion during the formation of these 
rocks. 
The analogies between the structure of Archean rocks and that 
of crystalline schists which have been produced by the metamor- 
hism of ordinary sedimentary formations have been already 
pointed out (Book IV. Part VIII.). The Archaan gneisses and 
schists are distinctly bedded, and their alternations of schists, quartz- 
ites, and marbles, closely resemble those of the shales, sandstones, 
and limestones of younger geological periods. The conglomerates 
above alluded to furnish unquestionable evidence of an original. 
clastic structure in some of the strata. The grains, streaks, layers, 
and thicker zones of graphite in the gneiss remind the observer of 
the way in which coaly matter is diffused through the sandstones 
and shales of the Coal-measures. Hence it is difficult to avoid the 
inference, that these ancient crystalline rocks represent former 
marine sediments. 
In one of the Archean (Laurentian) limestones of Canada, speci- 
mens have been found of a remarkable mixture of calcite and 
serpentine. ‘These minerals are arranged in alternate layers, the 
calcite forming the main framework of the substance, with the 
serpentine (sometimes loganite, pyroxene, &.) disposed in thin, 
wavy, inconstant layers, as if filling up flattened cavities in the 
calcareous mass. So different from any ordinary mineral segregation 
with which he was acquainted did this arrangement appear to Logan, 
that he was led to regard the substance as probably of organic origin. 
This opinion was adopted, and the structure of the supposed fossil 
was worked out in detail by Dr. Dawson of Montreal, who pro- 
nounced the organism to be the remains of a massive foraminifer 
which he called Hozoon, and which he believed must have grown in 
large thick sheets over the sea-bottom. This opinion was confirmed 
by Dr. W. B. Carpenter, who, from additional and better preserved 
specimens, described a system of internal canals having the characters 

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