TRANSACTIONS OF THE SECTIONS. 81 
their characters to those of the runs or tunnels of these animals, as observed by him 
on our northern shores. 
Having arrived at this conclusion, it is then suggested that these fossil tracks may 
have been produced by Trilobites, which agree very well in size with them. The 
largest tracks of the grooved form are a little above an inch wide. The width of the 
pygidium of Phillipsia truncatula, a carboniferous species, is 11 lines, that of the 
cephalic shield would probably be a little more; therefore if allowance be made for 
the thickness of the tunnel wall of the burrow or track, and the necessary enlargement 
of the calibre beyond the width of the animal, it is evident that, so far as the size is 
concerned, the largest tracks might be attributed to this species. 
The nodulous tracks are stated to be not more than half an inch wide; there can 
therefore, be no difficulty as to size with respect to this form, for several species of 
Trilobites are described to be about that size. And in conclusion it is remarked that 
the large cephalic shield with its anterior or head-tubercle (glabella) and projecting 
** cover of the eyes,” appears well calculated to plough its way beneath tlie surface of 
the sandy or muddy beach. 
Burmeister, indeed, in his work on the ‘ Organization of Trilobites,’ published 
by the Ray Society, expresses an opinion that their habits, like their structure, resem- 
bled those of the Phyllopoda (a tribe of the Entomostraca), and that they ‘‘moved 
only by swimming in an inverted position close beneath the surface of the water, and 
did not creep about at the bottom as Kléden supposed.” ‘Though their habits may 
have been similar to those of the Phyllopoda, there does not seem any good reason 
for asserting that there was no deviation in this respect. In fact, the organization of 
the two groups differs in so many particulars, that some variation in their modes of 
life might naturally be looked for. The Trilobites may have occasionally swum at 
the surface as supposed, and also have burrowed in the mud or sand at the bottom 
of the water or on the beach. Season, too, may have modified their habits in these 
respects. 
On the Distortion of Fossils. By Professor Harkness, F.G.S. 
On the Origin of the Breccias of the Southern Portion of the Valley of the 
Nith, Scotland. By Professor Harkness, £.G.S. 
The southern portion of the Vale of the Nith consists of deposits of the Permian 
age, capable of being divided into three groups. The lowest is composed of sand- 
stones which afford footprints, the middle of thick breccias, and the upper of red 
sandstone. The middle member is for the most part made up of angular fragments, 
which have been derived from the Silurian strata surrounding the Permian area, 
Besides these angular Silurian fragments, portions of red porphyries also occur in 
these breccias, and these portions of porphyries have an equally angular character 
with the Silurian fragments. 
The porphyritic pieces which are imbedded in these rocks have no representatives 
in the form of rocks of porphyry nearer than about twenty miles from the localities 
where they present themselves; and no ordinary action of water is able to account 
for their occurrence in an angular condition among the constituents of the breccias 
of this locality. 
Professor Ramsay has described the angular transported fragments which form the 
Permian breccias of Worcestershire and Shropshire (Quart. Journ. Geol. Soc. vol. xi. 
p. 185 eé seq). In these breccias the fragments have their surfaces, in many in- 
stances, well marked by grooves and striz, and they possess many features which 
have induced Professor Ramsay to regard the breccias of these counties as having 
resulted from the agency of ice, leading to the inference that Arctic conditions pre- 
vailed during a portion of the Permian epoch, 
The deposits which, in the southern portion of the Vale of the Nith, are composed 
of angular fragments, do not bear about them the same features in the form of grooves 
and strize which characterise the breccias described by Professor Ramsay. The 
angular nature of the fragments, and the distance which some of them have been 
transported, justify the conclusion that transport by water has not been the agent 
1858. 6 
