1848.| NICOL ON THE SILURIAN ROCKS OF THE TWEED. 205 
annelids, but have mislaid my specimens and have not had an oppor- 
tunity of procuring others. This paucity of fossil remains does not, 
however, imply a corresponding paucity of animal existence. The 
character of the rocks mineralogically is by no means well-adapted 
to preserve organic remains, and as the formation appears to have 
been chiefly deposited in a deep sea, where calcareous matter was by 
no means abundant, this may explain the rarity of testaceous mol- 
lusca. There are, indeed, in many parts of the formation, indications 
of organic bodies, or at least forms in the rocks which may be re- 
garded as such. Thus, in the Pirn Crag, in Peeblesshire, there are 
numerous ellipsoidal concretions which appear on the exterior sur- 
face, where it is beginning to decay, like the letter O distinctly carved 
in the stone. These concretions do not differ mineralogically from 
the rest of the rock, except that they are occasionally staimed brown 
by iron, and show no trace of structure ; yet it is probable that they 
occupy the place of some organic body, perhaps of a rolled-up trilo- 
bite. In the same place, and indeed throughout almost the whole 
greywacke formation, numerous concretions of more irregular forms 
occur. These principally appear when the rock begins to decompose, 
when they, wasting more readily than the mass in which they are 
imbedded, leave holes and cavities of various fantastic shapes. To 
these also I am disposed to ascribe an organic origm, and would 
regard them as representing the sponges or other soft coriaceous 
animals of the Silurian seas. : 
Though there is thus sufficient evidence that animal life was not 
wholly banished from these ancient waters, and was in some places, 
probably near calcareous springs, in considerable. abundance, yet 
from the character of the rocks but few species can be specifically 
determined with certainty, and can scarcely be regarded as positively 
fixing the true age of the formation. The specimens exhibited are 
indeed such mere fragments that I despaired of more than one or 
two species being capable of determination, and valued them chiefly 
as indications of the existence of animal life at that time, and as 
. holding out hopes of better success in further researches. Mr. Salter, 
whose great knowledge of Silurian fossils is well known, has kindly 
undertaken to compare them with the rich collection im the posses- 
sion of the Geological Survey, and has furnished me with the follow- 
ing list and remarks. 
Notice on the Fossils collected by Mr. Nicou in PEEBLESSHIRE. 
In the following list all the forms are noticed, and an attempt 
made to name them all, however imperfect—as the errors made will 
be easily corrected from more extended collection in the same locali- 
ties, and an indefinite reference of them to genera merely would help 
nothing in provisionally fixing the date of the rocks. 
TRILOBITES. 
Asaphus. Tails—very like young A. megistos, of the American 
Lower Silurian rocks. 
