11. L. Wells — Caesium-Mercuric Halides. 221 



Conclusions. — As a result of this investigation no change is 

 suggested for the Lower Devonian — the Cauda-galli grit and 

 Upper Helderberg (Corniferous limestone). In the Middle 

 Devonian the Marcellus is generally clearly defined, except in 

 fhe upper part where it changes rather gradually from the 

 argillaceous to the more arenaceous shales of the overlying 

 Hamilton stage. The Hamilton as mapped and defined con- 

 sists mostly of rather coarse arenaceous shales and thin sand- 

 stones ; but to these possibly should be added the calcareo- 

 arenaceous zone (called Tully limestone) and the black, 

 fossiliferous shales above (called Genesee). It is shown that 

 the so-called Tully and Genesee stages do not agree with the 

 New York formations in either lithologic or paleontologic 

 characters. After the disappearance of the Tully limestone 

 and Genesee black shale in central New York there is very 

 little evidence of their reappearance in eastern New York. 

 Therefore the correlation of the zones in Pennsylvania with 

 these New York formations is considered as open to discus- 

 sion. The Chemung series contains a modified Hamilton 

 fauna similar to that in the lower Portage of central New 

 York, after the disappearance of the Tully limestone and 

 Genesee shale. Above this fauna are the Starucca sandstones, 

 the New Milford red shales, the Delaware flags with Ortho- 

 nota (?) pa?'vida, and the Montrose shales in the upper part of 

 which Sjpirifera mesastrialis and Leda diversa {?) occur. 

 These fossils seem to indicate that up to this horizon this series 

 of rocks may not be of later geologic age than the lower part 

 of the Chemung stage. From the above statement the inabil- 

 ity to indicate any sharp dividing line between the Catskill 

 and Chemung series, or the Chemung and Portage stages, will 

 be readily understood. 



Acknowledgment is due to Mr. Charles D. Walcott, Chief 

 Paleontologist of the U. S. Geological Survey, and to Professors 

 H. S. Williams of Cornell University, I. C. White of West 

 Virginia University, and Lester F. Ward of the U. S. Geologi- 

 cal Survey for suggestions and advice in this work. 

 U. S. Geological Survey, May, 1892. 



Art. XXIX. — On the Caesium- Mercuric Halides '• by H. L. 



Wells. 



It is to be expected that more complete series of double- 

 halides can be made with caesium than with the. other alkali- 

 metals, because it is the extreme member of the potassium 

 group and the most electro-positive element known, and because 

 csesium double-salts in general are less soluble than the corre- 

 sponding compounds of the other alkali-metals. A thorough 



