216 Scientific Proceedings, Royal Dublin Society. 



olialk — sometimes those containing flints, known in the country by 

 the name opoka ; sometimes the green sands. 



The Silurian slates present two very distinct forms — the one 

 large, grained and compact, with rough surfaces, greenish-brown 

 colour, giving rise, by splitting, to thick flags ; the other, slaty in 

 its texture, is composed of thin, even, greasy, lustrous, and friable 

 plates of a greenish-black or, more rarely, greenish, colour. This 

 latter form is that occurring in Russian Podolia, and is character- 

 ised by its greater tendency to weathering, giving rise to more 

 rounded outlines of country. The first-mentioned clayslates produce 

 in the valley of the Dneister and its confluents very abrupt banks, 

 sometimes almost perpendicular. The thickness of these clayslates 

 is of several hundred feet. It is in the greenish-black and thin- 

 plated clayslates that the globular phosphorite is exclusively found, 

 these beds being, according to Bloede and Barbot, referable to the 

 Silurian formation. 



The phosphorite balls are inter stratified with these slates, often 

 in great quantities. Owing to the tendency to weathering of the 

 slates, the balls give rise to secondary deposits, such as the detritus 

 heaps of the chalk beds, and are found thus along the banks of 

 the Dneister, even far into the river bed. 



The localities marked by the more frequent occurrence of the 

 phosphorite balls are situated on the left bank of the Dneister, 

 in the zone of country lying between S*^^ TJszica and Mogilew. 

 Yery fine surface deposits are met with at Zurczewka, Kaljus, and 

 Ljadowa, 



The mineralogical characteristic of the Podolian phosphorite is, 

 that it occurs, almost without exception, in a spherical or globular 

 form, showing an inner concentric radiated structure. 



The surface is uneven, often foliated, feels greasy, and presents 

 a dark-green colour, somewhat like that of fresh cast iron. The 

 balls, disengaged from the matrix by erosion, and having under- 

 gone abrasion, present a bright-grey and even surface. 



The diameters vary from 2 cm. to 16-18 cm., generally 5-6 cm. 

 The density is from 2"8 to 3. The hardness about that of fluor- 

 spar. 



The radiated structure is not in all cases the same : in some 

 this structure is more marked towards the periphery, becoming in- 

 distinct towards the centre, where it appears compact ; the core 



