RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 191 



In the east, on the Lawa, Walehow, and other rivers of that district, 

 the limestone of the Walchow formation is rather soft, and disinte- 

 grates very readily on exposure, so that great numbers of very beauti- 

 fully preserved fossils may be obtained, especially in the extensive 

 quarries on both sides of the Walchow above Old Ladoga. 



The lowest limestone has a decidedly green color, due to the presence 

 of a considerable quantity of glauconite. The glauconite is in small 

 grains about .5 mm. in diameter, and makes a considerable part of the 

 limestone. In Esthonia the corresponding bed is much harder, is a 

 deeper green in color and contains more and larger grains of glauconite, 

 up to 2 mm. in diameter. 



All of the limestone in the formation on examination in thin section 

 proves to be made up almost entirely of fragments of fossils, largely 

 trilobites, but also ostracods, bryozoans, and brachiopods. All these 

 are in small pieces, sometimes 2 mm. long, but generally much less. 

 The cement consists of exceedingly small grains of crystalline calcite. 

 Stray grains of glauconite are seen in most of the slides from all horizons 

 in the formation. The red and brown colors of some of the limestone 

 prove to be due to a stain and grains of limonite surrounding the 

 crystals of the matrix and filling the zoecia of the bryozoans. The 

 limonite halo around the grains of glauconite suggest that the source 

 of the iron compound may possibly be found in the decomposition of 

 that mineral. (Plate 5). 



In a slide from Putilowa especially, very few of the glauconite 

 grains are unaltered, but almost all show a border of limonite, and 

 the grains contain much of a dark alteration product along cracks. 



The glauconite from the glauconite sand and limestone has been 

 analyzed by Kupffer (27), some of whose analyses are quoted below. 



99.40 100.00 100.69 100.00 



