4A Reéxamination of American Minerals. 
made obvious when we take the ratio between the silica and alu- 
ina, which in the purer varieties (analyses 3 and 4) is as 4: 1, 
and analysis 4 gives the ratio 12: 3-04: 1:11, or RSi+#Si*. 
he specimens examined were received from Professor Sil- 
liman, Jr., and Mr. Samuel W. Johnson. 
13. Danbury Feldspars ; 1. Oligoclase ; 2. Orthoclase. 
1. Oligoclase——The feldspar in which the Danburite occurs 
has so strong a resemblance to the oligoclase from Sweden that 
we have been led to analyze it; the results of our examination 
prove its identity with that species. 'The analyses gave ; 
Silica, ; ; 64-03 i ; 63°50 
ina ‘ : 22°37! ; : ; 22°75 
Peroxyd of iron, ‘ trace ; P ‘; trace 
Lime, ‘ ; 2°91 ; 8°28 
Magnesia, trace, trace — 
Soda, . ; 10°06 937 
Potash, “60 
Ignition, 30 21 
100°27 99°61 
These give the oxygen ratio 1:3: 9 and the formula RSi+A15i, 
which are the ratio and formula for oligoclase. 
less contain a little oligoclase that it is impossible to separate. 
1 
oo : 2. 
Bilica, J : : 63°80 3 ‘ 63°95 
Alumina, : ; : 18:90 ‘ ; A 19°05 
Lime, hey ; J 80 ‘ 61 
Magnesia, . é : "20 ‘ : : “20 
Potash, j i ‘ 11-43 ‘ : ; 10°95 
Soda, ee ; 3°69 
Ignition, . : 4 30 “50 
98°95 
The specimens examined were taken from the locality by one 
of the authors. 
14. Haddam Albite, identical with Oligoclase. 
Associated with the iolite at Haddam, Conn., there occurs a 
glassy feldspar which has heretofore been called albite. Its com- 
