326 abstracts: geology 



determined the boundaries of the test spot or "target," the length of 

 which could be varied by varying the slit width. Precautions were 

 taken to have the brightness across the image of the slit uniform. The 

 brightness of either the test spot or field could be varied by means of 

 sectored discs, so that any desired contrast between them could be 

 obtained for any given brightness of field. 



Curves are given showing the relation between the length of the 

 test spot and the brightness of the field for various contrasts, and also 

 curves showing the relationship between the length of test spot and the 

 contrast between field and spot for various values of the field bright- 

 ness. E. K. 



GUOLOGY.— The Porcupine Gold Placer District, Alaska. H. M. 

 Eakin. U. S. Geological Survey Bulletin 699. Pp. 28, pis. 8. 

 1919. 



The Porcupine gold placer district lies in the headwater region of 

 Chilkat River, near the British Columbia boundary, about 100 miles 

 northwest of Juneau, or 25 miles west of Skagway. Productive mining 

 began here in 1899 and continued so successfully that the district has 

 ranked as one of the most important placer fields in Alaska. This 

 district was visited by Geological Sur\^ey parties in 1899, 1903, and 1916, 

 and the present bulletin gives a summary of the knowledge thus ob- 

 tained. It includes a geologic map and several views of the district. 

 The total output to the end of 1916 is estimated at $1,200,000. 



The bed rock consists mainly of limestones, slates, and other sedi- 

 mentary rock, intruded by diorite of the Coast Range belt. Small 

 diabase dikes also intrude the bedded rocks locally. The placer gold 

 is derived mainly from mineralized belts of sedimentary rock carrying 

 small quartz and calcite veins and in which sulphide minerals are 

 abundant. R. W. Stone. 



GEOLOGY. — Our mineral supplies. H. D. McCaskey and E. F. 

 BuRCHARD. U. S. Geological Survey Bulletin 666. Pp. 278, 

 pi. I. figs. 6. 1919. 



This bulletin consists of short papers prepared and published in 

 191 7 on about 30 minerals to meet a demand from the public and from 

 other Government bureaus for information and advice concerning 

 these minerals. Each separate chapter discusses the source of supply 

 of the mineral, its uses both in peace and war, and the normal demand. 



