TITLES AND ABSTRACTS OF PAPERS 111 



A PLANE-TABLE FOR MILITARY MAPPING 

 BY ALAN M. BATE MAN 



(Abstract) 



During the teaching of military mapping at Yale University a plane-table 

 equipment was designed, at a low cost, for instruction in that particular phase 

 of mapping. It enables more effective results to be obtained than the equip- 

 ment used in most training centers and described in the text-books. 



Presented by title in the absence of the author. 



SAND-CHROME DEPOSITS OF MARYLAND^ 

 BY JOSEPH T, SINGEWALD, JR. 



(Ahstract) 



Though competition put an end to rock-chrome mining in Maryland at about 

 1880, there has been a small production of sand-chrome concentrates to the 

 present time. The output is exported to Europe, where it is used to set the 

 colors of painting on fine porcelain ware. The Maryland deposits are capable 

 of supplying this limited market indefinitely, but could not support a greatly 

 augmented production. 



Chemical analyses of several samples of the concentrates and of magnetic 

 and non-magnetic portions into which they were separated yielded some inter- 

 esting data concerning the characteristics of the spinel minerals found in the 

 chrome-bearing serpentine areas of Maryland. Following is a brief summary 

 of these results : 



A spinel molecule, carrying an equivalent of only 29 per cent magnetite, may 

 be highly magnetic if chromic oxide is in excess of alumina, whereas one carry- 

 ing an equivalent of 26 per cent magnetite may be not at all magnetic if the 

 alumina is in excess of chromic oxide. Since alumina reduces the magnetism 

 of the spinel molecule, these figures indicate that a chromite containing no 

 alumina might be highly magnetic if it contains an equivalent of less than 25 

 per cent magnetite. Such a chromite would appear to be magnetite in the field 

 and might be disregarded in a search for chrome ore, yet it would analyze 51 

 per cent CraOg. On the other hand, a non-magnetic black spinel in a chromite- 

 bearing region, which in the field would appear to be good chrome ore, might 

 run so high in alumina and iron that the chromic oxide content of the ore 

 would fall to less than 7 per cent. 



These data point out the inadequacy of simple field tests in prospecting 

 for chrome ore and the necessity of checking such observations by chemical 

 analyses. 



Read in full from manuscript. 



1 Presented with permission of the State Geologist. 



