C. Ludehing — Synthesis of Crocoite and Phomicochroite. 57 



cephalus) Chippewensis = P. (L.) minor, Amphion matutina, 

 and Agraulos thea. 



The line of demarcation between the Upper and Middle 

 Cambrian in Wisconsin is drawn between the beds carrying 

 the. preceding fauna and the superjacent strata. These two 

 horizons also occur in the Potsdam sandstone about the Adi- 

 rondack Mountains and in the Cambrian section of the Llano 

 Hills of Texas. 



It was not until after the recent work in the Cambrian of 

 Tennessee, Alabama and Georgia and an extended comparison 

 of the faunas with those of the Upper Mississippi Valley was 

 made, that the line of demarcation between the Middle Cam- 

 brian and Upper Cambrian was drawn. The reasons for this 

 will be more fully presented in a memoir now being prepared 

 upon the Middle Cambrian rocks and faunas. 



In this preliminary study I had the use of the unpublished 

 geologic maps of eastern Tennessee that have been prepared 

 by the Appalachian Division of the Geological Survey, and 

 Mr. Bailey Willis gave me great assistance by placing at my 

 service his extensive knowledge of the country and its geol- 

 ogy. It is anticipated that the work on the Cambrian rocks 

 and fossils will be continued in the Appalachians during the 

 field season of 1S92. 



Art. VII. — Synthesis of the minerals Crocoite and Phae- 

 nicochroite • by C. Ludeking, Ph.D. 



The synthesis of Crocoite and Phcenicochroite may be accom- 

 plished by exposing for several months to the air a solution of 

 lead chromate in caustic potash in a flat dish or plate. It is 

 possible thus to obtain a mixture of the crystals of the two. 

 Without any difficulty whatever the individual crystals can be 

 picked out separately by means of a pincette and obtained in a 

 state for analysis. 



The following are the analytical results obtained : 



Crocoite. 



Found, 



Pb 63-9 

 CrO, 35-2 



4 



Calculated. 



64-04 + 

 35-96 — 



Ph wn icochroite. 



99-1 



Found. 



Pb 71-2 

 CrO, 25-9 



100-00 



Calculated. 



71-43 — 



26-73 + 



It appears therefore from this showing that the chemical 

 composition of the artificial crystals approximates quite closely 



