278 The American Geologist. November, 1900 
IV. Theoretical composition of Mesolite of the formula: 
(Na2Al8Si30io+2HgO)+2(CaAl8Si30.io+3H20,o) or MviSco 
Mesotype Scolecite 
V. Theoretical comoosition of the compound: 
3(Na2Al,Si«Oio+2H,0)+7(CaAl,Si30io+3H20) or My^Sc, 
VI. Gonnardite from Gignat. Analysis by Pisani, corresponding 
lo the formula: 
(Ca, Na3)aAloSi,-,Oi j+S-SHoO. See Lacroix: Mineralogie de la France 
II, p. 279, 1896. 
VII. Column II, after adding the K-O and Na^O, calculated to 
tlie scale of 100. 
VIII. Column 11, after deducting the water, calculated* to tlie 
scale of 100. 
The composition agrees almost exactly with that expressed 
l)y My^ScT, and it is probable that it shotild be considered as an 
isomorphous compound. In this connection it is interesting 
to note that, while so far as known the optical properties of 
these minerals are not such as are characteristic of isomor- 
phous series, a careful examination of the analyses already 
published will disclose a series nearly complete, from meso- 
type to scolecite. 
Pseudomesolite may be readily distinguished from meso- 
lite by the following characters: 
First. Its optical orientation, ng being nearly parallel to 
the elongation, which is therefore always positive (in mesolite 
//m is nearly parallel to the elongation). 
Second. Its very small optic angle. 
Third. Its high maximum extinction angle of at least 20° 
in the zone of the vertical axis. 
Fourth: Its weaker birefringence. 
The chemical composition of pseudomesolite is distinctly 
different from that of gonnardite,'y though the optical differen- 
tiation of the two minerals is somewhat dif^cult. The bire- 
fringence of pseudomesolite is, however, only about one-half 
of that of gonnardite, while the extinction angle in sections of 
the vertical zone reaches a maximum near 20° in pseudomeso- 
lite and is always 0°, or very near it, in gonnardite. 
Pseudomesolite is not readily confused with any other zeo- 
lites; in exterior characters it somewhat resemblesthomsonite, 
*See Plate XIII, Figure 12. 
tSee Lacroix: Mineralcgie de la France. II. 1896. p. 279. 
