W. G. Fearnsides—Lower Ordovician Rocks of Scandinavia. 299 
district become shaly, and with Ogygia dilatata and other Llandeilian 
forms contain Didymograptus geminus. 
The Boundary between Cambrian and Ordovician Systems. 
Summarily, then, we may say that the main stratigraphical features 
of the Scandinavian Tremadoc and Arenig rocks are inconsiderable 
thickness and heterogeneity. Their lowest beds, like other members 
of the Alum Shale Series, preserve a character so uniform that beds 
hardly more than a foot thick can be recognized in districts five 
hundred miles apart. The question of the contemporaneity or 
otherwise of Dictyonema shales with parts of the Ceratopyge shales 
remains, but the equivalence of Dictyonema shales and Obolus con- 
glomerate seems practically certain. The existence of the Obolus 
conglomerate interstratified among Dictyonema shales, the break 
between Dictyonema or Ceratopyge shales and overlying glauconitic 
beds, and the numberless lines of contemporary erosion within the 
latter, all seem to preclude the possibility of any of these beds having 
had a truly deep-water origin. The wide distribution of individual 
species of organisms and the comparative thinness of the beds must, on 
the other hand, most certainly point to a state of great uniformity, and 
possibly to very open-water conditions. The replacement of the 
graptolitic Arenig mudstones of the west by the comminuted calcareous 
organic deposits which form the eastern limestones with their ripple- 
marks and worm-tracks is further evidence in the same direction, 
and would seem to show that the eastern districts were at that time 
areas of considerable tectonic instability. All these stratigraphical 
considerations have doubtless had their influence in determining for 
the Swedes, who in such matters are essentially a nation of paleeon- 
tologists, at what horizon they shall separate the first or Cambrian 
from the second or Ordovician system, and as the facts have 
accumulated but slowly, a settlement of the question has been equally 
gradually evolved. A brief historical summary of the process may 
perhaps be of interest. 
In 1854 Angelin,! following Barrande’s idea that geological systems 
are to be defined as definite stages of paleontological evolution, 
determined that rocks containing a predominance of Olenid trilobites 
shall in Sweden be termed Cambrian, while those with a corresponding 
abundance of Asaphids shall belong to the Lower Silurian (Ordovician). 
In 1869 Linnarsson (W 1) further developed the same idea, and 
placed the ‘‘ Regio Ceratopygarum” within the Ordovician. About 
the same time also Linnarsson began to make use of the graptolites to 
mark subdivisions within systems already constituted, and in so doing 
decided to group the Dietyonema beds, which at that time were not 
known to contain trilobites, with the lithologically similar Alum 
Shales of the Cambrian. In 1881 Brogger(K 1) in Norway, following 
Kjerulf and Linnarsson, placed the Ceratopygekalk (3a y) within the 
Ordovician, and on the evidence that underlying Ceratopyge shales (3a 8) 
and beds ‘with Symphysurus incipiens (3aa) both contain Asaphid 
trilobites, grouped the three together as Ceratopyge beds at the base 
of the Ordovician, and again left the Dictyonema beds as the highest 
1 N. P. Angelin: ‘‘ Paleontologia Scandinavica,”’ 1852, 1854. . 
