266 W. G. Fearnsides—Lower Ordovician Rocks of Scandinavia. 
orsten bed with a matrix of glauconitic shale or limestone containing 
a Ceratopyge fauna. Such a phenomenon is very well seen at Nya 
Dala and Kkedalen in Southern Falbygden (W 5), at Mosseboo under 
Hunneberg (W 6), and at Borgholm in Central Oland (0 5). At Dala 
a pair of fine flat orstens some 7 feet in diameter and crowded with 
Peltura scarabeoides were seen to overlie some 3—7 inches of glauconiti¢ 
shale with Orthis Christiant for a length of about 6 feet. At 
Mosseboo the upper surface of a calcareous bed with Spherophthalmus 
is so brecciated and the basal Ceratopygekalk so dark and shaly that 
the line of junction is difficult to find in even the cleanest of hand- 
specimens. Such difficulty, however, is most unusual, and the base 
of the Glauconiteskiffer, however irregular, is almost always clean- 
cut, and the topmost Alum Shales so little weathered that either 
in soft shale or hard orsten the Alum Shale fossils are preserved and 
recognizable within a twentieth of an inch of the boundary. All these 
characteristics of the Glauconite shale and Ceratopygekalk it shares 
with most Greensand deposits, and especially with such formations as 
the Cambridge Greensand, which follows a period of subaqueous 
erosion. ‘he phenomenon of Erosion-pocketing or Korrosionsgruppar 
is more peculiar, and, so far as I can learn, almost unique among 
known sedimentary rocks. It is undoubtedly an erosion phenomenon, 
and as such is difficult to describe. It has been already discussed 
by Anderson in his well-known monograph of the Phosphate deposits 
of Sweden,' but no English notice of it seems to have appeared, and 
no satisfactory explanation of its mode of formation is forthcoming. 
The most striking museum specimens of Korrosionsgruppar are 
obtained from the base of the Glauconiteskiffer, where the rock is 
sufficiently calcareons to hold together, and rests upon a bed of orsten 
or Alum Shales also firm enough not to be friable. Such a specimen 
from Ulunda in Falbygden is depicted on Plate IX in Figs. 1 and 2, 
which show photographs of two sides of the same block taken at right 
angles. The figures are photographed natural size. The lower part 
of the block is a homogeneous, fine-grained, and somewhat earthy orsten 
of the Peltwa zone, and, as is well shown by the photograph of the 
under-surface, Fig. 3, and by the projecting lines of trilobite fragments 
seen in Figs. 1 and 2, retains its bedding quite undisturbed. The 
junction with the glauconitic Ceratopygekalk is absolutely sharp, but 
quite irregular, and it may be seen that the irregularities are produced 
by a process of pitting rather than of grooving or moulding. The pits 
are generally round or oval and rather pocket-shaped, but never more 
than an inch or two deep, and though their mouths may vary from the 
size of a lead pencil to that of a five shilling piece, the diameter near 
the bottom is rarely bigger than a sixpence. The sides of the pits may 
be vertical, sometimes slightly overhanging, but they are more often 
inclined one towards the other downwards, and the depth is by no 
means proportional to the diameter of the opening. The bottom of 
the holes is usually somewhat rounded, and is often almost hemi- 
spherical. In general the lip of the pit is rounded, but in some cases 
sides of adjoining pits meet in a cusp whose solid angle may be only 
some 30°, but in such cases I think that the pits must be of different 
1 Bull. Geol. Inst. Upsala, 1895. 
