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characteristic boulders, which cannot be placed in the Eastern baltic 
scheme; these, together with others whose age lies between limits 
too far apart to reduce them to one of these divisions, will be 
described at the end. 
After these introductory remarks we may proceed to the 
description of the boulders of the first-mentioned group. 
G,—G,. 
The boulders belonging to the oldest zone G',, those of the Jorden 
beds in Esthonia with Leptocoelia Duboisii pe VerN., which are 
occasionally mentioned by German geologists, are not found near 
Groningen. The two younger zones G, and G,, however, have 
been met with. 
29. Borealis-limestone. 
These well-known and characteristic boulders consist of limestone 
or dolomite, and usually contain in large quantities remains of 
Pentamerus borealis Ercuw., 
while other fossils are absolutely wanting. As regards the kind of 
rock my material from Groningen may be divided into two varieties: 
a. Limestone, as a rule distinctly crystalline but somewhat marly, 
as may be easily observed on its weathered surface. The slightly 
variegated colour of the ground-mass shades from gray to brownish- 
yellow at the fresh fracture; if weathered, however, it has mostly 
a sallow-vellowish-gray tinge. In this ground-mass the valves of the 
above-mentioned species of Pentamerus always occur in great numbers; 
they are invariably changed into crystalline calcite and this is very 
often of a bright white colour, so that the always very thick shells 
sometimes stand out very distinctly against the surface of the boulders, 
which is sometimes polished. Besides the ground-mass weathers more 
readily than this calcite, so that the fossils appear in relief. The 
number of these petrifactions has influenced the exterior appearance 
of the boulder. Though always numerous, the ground-mass may yet 
occur in sufficient quantities to give a compact character to the stone, 
These limestones which are rather hard when not partially weathered 
make up the majority of the stones found. The dimensions of some 
of them amount to about 17 ¢.M. In other pieces the ground-mass 
recedes much to the background and the stone consists almost 
exclusively of fragments of the valves of this species of brachiopoda 
