RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 197 
duction of Echinosphaerites and Chasmops, and the acme of the varia- 
tion of the genus Asaphus. Echinosphaerites is not found in the 
Linsenschicht at the base of the formation, though it does oecur with 
linsen a few feet above the base at some localities (Ontika and As- 
serien). 
The soft limestone from St. Michael Archangel, on the Walchow, 
weathers to a nearly white flour which, when wet, forms a very sticky 
mud. A thin section shows that this rock is made up almost entirely 
of very small fragments of fossils, few of which reach 1 mm. in length 
and none have more than one fourth that thickness. The most 
abundant fragments are of some organism with minute tubules, 
possibly a Solenopora. Bryozoa, Ostracoda, and trilobites seem to 
furnish a large part of the material. The fragments are much more 
finely comminuted than in the limestone of the Walchow. 
Characteristic fossils are: — Echinosphaerites aurantium, Clitam- 
bonites adscendens, Porambonites aequirostris, Chasmops nasuta, Cerau- 
rus exsul, Illaenus tauricornis, Asaphus cornutus, and A. kowalewski. 
Reval formation. Cig (upper part of Echinosphaerites limestone) 
of Schmidt. 
Resting upon the Dubowiki at Dubowiki on the Walchow, and 
through the greater part of Esthonia, and upon the Upper Linsen- 
schicht from Reval westward, is a hard, compact, sparingly fossili- 
ferous limestone, frequently magnesian in character, to which the 
name Reval may be applied, as it is very extensively quarried at that 
locality. The thickness and lithological character of this formation 
are remarkably uniform all the way from Baltishport to Dubowiki 
and it is a favorite quarry rock wherever accessible. The beds vary 
in thickness from an inch to about a foot and afford both building and 
flagging stone. It is extensively used for both purposes in Reval, 
Narwa, and Petrograd. Certain of the layers are traversed by vertical 
tubes suggesting worm-burrows. The thickness varies from twenty- 
five to thirty-five feet. Fossils are not very common, and in many 
eases dolomitization has gone on to such a degree that the rock has a 
porous appearance and the fossils are represented by hollow molds. 
The rock has about the same color and appearance as the Galena 
of Minnesota, and there does not seem to be much question but that 
the dolomitization has here taken place in beds originally composed 
mostly of limestone. In thin section the rock from Dubowiki shows 
irregularly intergrown areas of very small crystals of calcite with 
irregular boundaries, and areas in which the crystals are of dolomite, 
about twice the size of those in the areas of calcite, and with definite 
