Geology of the Lake Superior Land District. 25 
that they can be clearly recognized and defined as belonging to 
different ages. 
De la Beche remarks that, although alterations in the mineral 
character of the fossiliferous rocks, from the influence of intruded 
igneous matter in a molten state, or arising from other modifying 
causes, often produce mica slates, hornblende slates, gneiss and 
other forms of laminated and stratified deposits, with a peculiar 
aspect, there appears, nevertheless, evidence in Scandinavia and 
the British Isles, and also in other parts of Europe, to show that, 
beneath all the fossiliferous rocks, there are mica and chlorite 
slates, quartz rocks, crystalline limestones, gneiss, hornblende, 
and other rocks of earlier production. ese may be, indeed, 
merely altered, or metamorphosed detrital and chemical deposits 
of earlier times, and possibly organic remains may eventually be 
discovered in them ; but until this shall happen, it seems desirable 
to keep them asunder, for the convenience of showing previous 
accumulations to those known as the Cambrian group. He, there- 
fore, proposes the name of Mona Series for the reception of these 
older rocks, which are well displayed in the island of Anglesea, 
in connection with those of the succeeding group. 
In the admirable and detailed examinations of the Geological 
Survey of Wales, where the numerous intercalated beds of trap- 
pean rocks and the complicated series of faults have rendered the 
task of unravelling the geology one of great difficulty, the sur- 
veyors have cléarly shown the existence of this azoic series below 
the lowest Silurian strata, which is there represented by the sand- 
Stone of Barmouth and Harlech. 
Barrande, also, in his investigations of the Bohemian basin, 
has recognized a series of semi-crystalline slates alternating with 
Compact argillaceous slates, below the lowest Silurian strata, in 
Which he has failed to detect any trace of organic life ; hence he 
has applied the name azoic to these rocks without meaning to 
assert positively that the series is absolutely destitute of all traces 
of life, but simply as indicating the great fact that, thus far, none 
have been discovered. 
# * * * 
LOWER SILURIAN SYSTEM. 
We now proceed to a description of the palzozoic series of this 
region. Unlike the rocks which we have hitherto described, they 
exhibit few traces of igneous outburst, and few displacements of 
the strata ; but, on the other hand, they repose nearly horizontally 
peace basset edges of the slates, or occupy the depressions in the 
ite. 
This general remark, however, must be received with some 
qualification, for we find that there existed, during a part of the 
* Gevlogical Observer, pp. 31, 32. ‘i : 
Szcoxp Series, Vol. XVII, No. 49.—Jan., 1854. 4 
