HYDROUS SILICATES—ZEOLITE SECTION. 297 
II. ZEOLITE SECTION. 
The species of the Zeolite Section have been described as 
having some relation to the feldspars in constitution. In 
the feldspars, as explained on page 273, the following ratios, 
for the protoxides, alumina, and silica which analyses af- 
ronerocours Lod, J+3: 6, 02328, 123: 9, 133: 10, 123: 12, 
So, among the zeolites, if the water be left out of considera- 
tion, these are the ratios: 1:3:4 (in Thomsonite), 1:5:6 
(Natrolite, Scolecite, etc.), 1:3:8 (Analcite, Chabazite, 
etc.), 1:3:10 (Harmotome), 1:3:12 (Stilbite, Heulandite, 
etc.). This fact, added to the absence or nearly total ab- 
sence of magnesium and iron, and presence instead of Nay, 
K,, Ca, Ba, make out a distinct relation to the feldspars, 
whatever may be the part which the water sustains in the 
compounds. Besides barium, strontium is sometimes pres- 
ent, an element not yet known to characterize a species of 
feldspar. | 
These minerals were called zeolites because they generally 
fuse easily with intumescence before the blowpipe, the term 
being derived from the Greek zeo, to Jdoil. Among those 
described beyond, Heulandite and Stilbite, have a strong 
pearly cleavage, and the latter is often in pearly radiations ; 
Natrolite, Scolecite, are fibrous and radiated, or in very 
slender prisms ; Thomsonite occurs either radiated, or com- 
pact, or in short crystals ; while Harmotome, Analcite, and 
Chabazite, and the related Gmelinite, occur only in short 
or stout glassy crystals, those of chabazite looking some- 
times like cubes. 
The zeolites are sometimes called trap minerals, because 
they are often found in the cavities or fissures of amygda- 
loidal trap as well as related basic eruptive rocks. Yet 
they occur also occasionally in fissures or cavities in gneiss, 
granite, and other metamorphic rocks. They are not the 
original minerals of any of these rocks; but the results of 
alteration of portions of them near the little cavities or fis- 
