300 W. G. Fearnsides—Lower Ordovician Rocks of Scandinavia. 
member of the Cambrian. In 1890 Moberg,' having found the 
Dictyonema shales of Oland passing upwards imperceptibly into 
Shumardia shales with Ceratopyge fauna, tried to include these 
Ceratopyge shales along with the Dvzctyonema beds in the Cambrian, 
and to begin the Ordovician above the lithological break or uncon- 
formity at the base of the Glauconiteskitter. More recently, 
however, he discovered the bands of Alum Shales with Ceratopyge 
shale fauna interstratified among Ceratopygekalk above the break, 
and having also in 1898 discovered the Ordovician types of trilobites 
within the Dictyonema shales of Sandby (S83), has been compelled to 
abandon that view. In 1900, therefore, he states (04) that the 
Dictyonema shales are merely a graptolitic facies of the Ceratopyge- 
skiffer, and as such must with them be included within the Ordovician. 
This seems, moreover, to be the view now held by all Scandinavian 
geologists, and if we can agree (1) that there are no true natural 
boundaries of worldwide application, (2) that geological systems at 
first broadly suggested may become on further knowledge mere 
matters of convenience, (3) that the best boundaries are those 
conventional faunistic lines which are most easy to define and trace, 
we too shall probably adopt the same. By adopting the Dictyonema 
horizon: we give ourselves the opportunity of utilizing to the full the 
earliest known readily recognizable fossil of pelagic and probably 
planktonic habit which is known to be preserved among all classes of 
sediment and in many and distant parts of the world, while by 
grouping the Dictyonema beds with the Ordovician we exclude any 
considerable developments of graptolites from the Cambrian, and place 
a horizon which for multitudinous profusion of graptolite individuals 
is unsurpassed in the system of graptolitic predominance. If, however, 
we continue to maintain that the old ill-constituted Tremadoc Slates 
must remain a series either at the top of the Cambrian or base of the 
Ordovician we become involved in all sorts of difficulties, and are 
quite unable to ascertain any corresponding boundary either up or 
down among these well-studied Scandinavian rocks. 
Comparison with British Rocks. 
Having now stated generally the main features of the Scandinavian 
Cambro-Ordovician succession, I propose briefly to compare our British 
rocks with the corresponding members of that succession, and in so 
doing will consider British rocks also in the order of their superposition. 
The newest Welsh rocks comparable with the Alum Shales are the 
Upper Dolgelly Group? or Black Band of the Upper Lingula Flags. 
These include the zone of Spherophthalmus and Orthis lenticularis 
below and the zone of Peltwra scarabeoides above. They are not more 
than ten times as thick as their Scandinavian equivalents, and except 
for cleavage and absence of calcareous (orsten) material might very 
well be the hardened representative of beds with Scandinavian 
lithology. ‘heir faunistic assemblages and succession are also similar, 
and they preserve their character unchanged along the whole of their 
1 <“Qm en afdelning inom Olands dictyonemaskiffer sasone motsvarighet till 
Ceratopygeskiffern in Norge”? : S.G.U., ser. C, No. 109. 
2 Belt: Grou. Mae., 1867, p- 538. 
