‘ 245 
led to a discussion, several of the members dissenting from 
Prof. Wurtz’s view, as not in accordance with any analysis 
published of these rocks, and the question was finally referred 
to me, to be decided by analysis and determination of alkalies. 
Treceived from Profs. Wurtz and Newberry several specimens, 
the investigation of which indeed, seems to prove the above 
statement to be correct. 
I communicate my analyses herewith, and the way in which 
I propose to calculate the results, believing them to be of in- 
terest to other parties also. Dana, in his Mineralogy, fifth 
edition, page 351, cites six analyses of felsites, which corres- 
pond in composition very nearly to the samples analysed by 
me. These samples were all granular, and feldspar could be 
readily distinguished in them under the microscope; number 
three contained mica also; this was a coarse variety and is 
employed largely as a building stone. Number five is shale. 
The analyses were made by fusing the finely-pulverized rocks 
with a mixture of carbonate of soda and potassa, and deter- 
mining in the fused portion SiO,, Al,O,, Fe,Os, CaO, MgO. 
The amount of manganese they all contained was too small 
to be determined. Water was determined by heating in a 
closed platinum crucible, and noting the loss; alkalies were de- 
termined by heating in a platinum crucible with carbonate of 
lime and sal-ammoniac, and proceeding in the usual way. 
The alkali consisted principally of soda, though traces of 
potassa (and lithia, in number three) could be detected by 
the spectroscope. 
ie 2. 3. 4, 5. 
SiO; = 77-70 80°53 73°79 75°95 44:88 
Al,Os = 11°81 9°92 11°32 9°52 20°29 
Fe.,0; = 1:89 1:99 3°75 0°26 16°46 
CaO = 0°55 0°63 1:67 0:03 5°10 
MgO = 043 ' 0°63 1°32 0:22 5°20 
NaO = 6°89 5:67 6:90 3°62 2°42 
HO = 0°82 1:14 1:24 3°79 5°55 
Mn,0; = trace trace trace 0°28 trace 
Cr.03 = trace 0°62 
CuO = 6°52 none 
10009 10051 99°99 10019 = 100-12 
