RAYMOND: CORRELATION OF THE ORDOVICIAX 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 occur with 

 Hnsen a few feet above the base at some locaUties (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- 

 honites adscendcns, Porambonites aequirostris , Chasmops nasuta, Cerau- 

 rus exsul, lUaenus fauricornis, Asaphus cornutus, and A. kowalewski. 



Reval formation. Ci^j (upper part of Echinosphaerites limestone) 

 of Schmidt. 



Resting upon the Dubowiki at Dubowiki on the ^Yalchow, 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, 

 Xarvva, 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 

 cases dolomitization has gone on to such a degree that the rock has a 

 porous appearance and the fossils are represented by holl5w 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 



