490 
EX.EMENTS OF GEOLOGY. 
of the primordial genus DiJcelocephalus 
(Fig. 581). On the banks of the St. 
Lawrence, near Beauharnois and else¬ 
where, many fossil foot-prints have been 
observed on the surface of the rippled 
layers. They are supposed by Professor 
Owen to be the trails of more than one 
species of articulate animal, probably al¬ 
lied to the King Crab, or Limulus, 
Recent investigations by the natural¬ 
ists of the Canadian survey have ren¬ 
dered it certain that below the level of 
the Potsdam Sandstone there are slates 
and schists extending from New York to 
third diameter. A lar^e Newfoundland, Occupied by a series of 
group. Potsdam sand- trilobitic lorms Similar in genera, though 
stone. Falls of st. Croix, species, to those found in the Eu- 
ropean Upper Cambrian strata. 
Huronian Series. —Next below the Upper Cambrian occur 
strata called the Huronian by Sir W. Logan, which are of 
vast thickness, consisting chiefly of quartzite, with great 
masses of greenish chloritic slate, which sometimes include 
pebbles of crystalline rocks derived from the Laurentian for¬ 
mation, next to be described. Limestones are rare in this 
series, but one band of 300 feet in thickness has been traced 
for considerable distances to the north of Lake Huron. Beds 
of greenstone are intercalated conformably with the quartz- 
ose and argillaceous members of this series. No organic re¬ 
mains have yet been found in any of the beds, which are 
about 18,000 feet thick, and rest unconformably on the Lau- 
rentian rocks. 
LAURENTIAN GROUP. 
In the course of the geological survey carried on under the 
direction of Sir W. E. Logan, it has been shown that, north¬ 
ward of the river St. Lawrence, there is a vast series of crys¬ 
talline rocks of gneiss, mica-schist, quartzite, and limestone, 
more than 30,000 feet in thickness, which have been called 
Lauren tian, and which are already known to occupy an area 
of about 200,000 square miles. They are not only more 
ancient than the fossiliferous Cambrian formations above de¬ 
scribed, but are older than the Huronian last mentioned, and 
had undergone great disturbing movements before the Pots¬ 
dam sandstone and the other “primordial” or Cambrian 
rocks were formed. The older half of this Laurentian series 
is unconformable to the newer portion of the same. 
Fig. 581. 
