o8 ο REPORT— 1850. 
powerful masses of semi-crystalline schist (a great thickness of the lowest deposits 
being azoic or without fossils), covered by other masses of argillaceous schist, conglo- 
merates and quartzose rocks (quartzites), alternating with each other and occasionally 
swelled out by contemporaneous igneous rocks. Whilst little or no calcareous matter 
occurs in the lower division, limestone constitutes nearly the whole of the upper 
Silurian rocks, argillaceous schists appearing principally at their base and summit. 
Notwithstanding this contrast in the petrographical characters of the two divisions, 
it is the decided conviction of M. Barrande, that they form but one uninterrupted 
sequence of deposits; for all the formations exhibit a conformable stratification 
throughout their succession, and the two halves of the basin are arranged with a 
synclinal inclination in reference to its longitudinal or major axis. 
The palzontological distinctions between the two divisions are scarcely less stri- 
king ; since in each we find a special fauna which alone distinguishes them sufficiently 
if compared in their general character. M.Barrande has, however, discovered a suf- 
ficient number of species, common to his upper and lower Silurian rocks, to prove, 
that in Bohemia they are as much united in one natural system as in England. He 
estimates that already 40 to 50 species (possibly many more) are common to the two 
divisions. 
The manner in which the species common to the upper and lower Silurian rocks 
occur, has occasioned M. Barrande to point out a new and remarkable fact. At 
about 3600 feet below the limit between the upper and lower Silurian rocks, and 
therefore at a considerable depth in that part of the lower division, which he says 
represents both the Caradoc and Llandeilo formations of Britain, he discovered a 
mass of peculiar strata regularly and conformably intercalated in the vertical series. 
They differ entirely in their lithological nature and in their fauna from those of the 
lower Silurian division, in which they are enclosed. These beds, having a geographical 
range of from 2000 to 3000 feet only, and parallel to the axis of the basin, reoccur 
symmetrically on each side of that axis; so that they constitute a large, lenticular- 
shaped mass, whose maximum thickness is about 300 feet. Now, the rocks which 
compose these strata are exactly the same as those which form the base of the upper 
Silurian division ; i. 6. black schists with graptolites, alternating with trappzan layers, 
and containing many spheroidal concretions of black limestone. Further, all the 
fossils of this mass are identical with those which characterize the base of the upper 
division, and are almost entirely the species common to the two divisions. None of 
the forms which really characterize the lower division are present, though that fauna 
is abundantly developed both above and below this interpolated mass. It also ap- 
pears, that no mixture has taken place between the animals belonging to the insu- 
lated mass and those of the inferior division which surround it. 
M. Barrande has given the name of “‘ colony” to the peculiar fossils thus buried at 
great depths in the lower Silurian rocks —a denomination which at once indicates his 
opinion of their origin. He conceives, that the animals deposited in the interpolated 
graptolite schists have been derived from a centre foreign to the Silurian basin of 
Bohemia. Having existed fora lapse of time in seas lying to the N.W., these animals 
were, he conceives, introduced into Bohemia by favouring circumstances, such as cur- 
rents carrying with them calcareous mud. When these conditions ceased, the asso- 
ciated animals perished. Their remains were then covered by deposits of an entirely 
different nature, enclosing the relics of creatures inhabiting the surrounding Bohemian 
sea, whose aborigines regained possession of all the area, from part of which they 
had been so long excluded. After a lapse of time represented by 3600 feet of marine 
deposit, this ancient or lower Silurian fauna was itself entirely destroyed by sudden 
eruptions of igneous matter which spread out over the tract. The sea of Bohemia 
being deserted after this Joca/ revolution, was re-inhabited by a new immigration, 
proceeding from the same centre of creation which had furnished the colonies of a 
former era. But on this occasion the new comers occupied all the Silurian sea in 
Bohemia, and continued to propagate during the whole of the deposit of the upper 
Silurian rocks; constituting a very emblematic and separate fauna, in harmony with 
the corresponding divisions of England, Sweden, &c.*. 
The fact of the Silurian colonies of Bohemia leads to the belief, that in the older 
* See the final views of the classification of the Silurian rocks, by Murchison, Journ. 
Geol. Soc. Lond., vol. i. p. 467; and especially in the work Russia in Europe, vol. i. chap. 1. 
