Optical Examination of several American Micas. 55 



ture which the hyperborean dew points could compress from 

 them, first came in contact with the surface of the earth, and, 

 consequently, with an evaporating surface, when they were 

 first felt as trades, and where, therefore, they were dry winds. 

 Now, could the vapour taken up by these winds so increase 

 the saltness of this sea in the trade-wind region, as to make 

 the water there, though warmer, yet superficially heavier than 

 that below, and also than that within the regions of variabl6 

 winds and " constant precipitation." If so, might we not 

 have the anomaly of a warm under-current in the South 

 Atlantic Ocean, for that was the only place of escape for a 

 counter current from the Atlantic ? — Proceedings oftheAmeri* 

 can Association, Charleston. 



Optical Examination of several American Micas. By B. SlLLl- 

 MAN, Junior, A.M., M.D., &c. 



(Continued from p. 345.) 



In the accompanying tables we have given for each speci- 

 men measured, its locality, colour, the source whence received, 

 the angle between the optic axes, and other observations 

 bearing on their history. It may afford a more correct im- 

 pression of the present state of this investigation to remark 

 that specimens from over one hundred American localities of 

 mica have been examined besides a considerable number of 

 uncertain locality ; and from many of these localities very 

 numerous specimens have been measured. Thus there have 

 been measured of muscovite specimens from about 50 locali- 

 ties ; of phlogopite over 30 ; of euphyllite 2 ; of margarodite 

 1 ; of biotite about 12 ; and of doubtful species 3 or 4. 



Biotite. — The number of localities in the United States 

 furnishing uniaxial mica (biotite) has been constantly di- 

 minishing since the commencement of these investigations, 

 as increased skill in observation and improved means of ex- 

 amination have shewn one after another of the reddish and 

 yellow or brown coloured micas to have more or less decidedly 

 the characters of phlogopite. 



Thus the dark brown mica of Fine, the red micas of 



