28 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
this group, with affinities toward prehnite. Its composition is fairly 
expressed by the formula 
Al 2 (SiO t ) 3 CaMgH 2 
which is that of prehnite with CaMg in place of Ca 2 . The integrity of 
this species, however, is not yet fully established. 
III. THE FELDSPARS AND SCAPOLITES. 
Although orthosilicic and trisilicic acids are technically distinct, and 
from a chemical point of view should be studied separately, their salts 
containing aluminum occur in such a variety of mixtures that in several 
groups of minerals the two acids must be considered as mutually 
equivalent, and their compounds discussed together. Two such groups, 
closely allied, are the feldspars and the scapolites. 
For each of these groups the theory developed by Tschermak has met 
with general acceptance. In the case of the feldspars, Tschermak was 
undoubtedly anticipated in great part by Hunt, Waltershausen, and 
others, but he gave to the theory its most precise statement, and to him 
its final establishment is due. According to this theory the triclinic 
plagioclase feldspars consist of albite, ALNaSi 3 8 , and anorthite, ALCa 
(Si0 4 ) 2 ; which, commingled in various proportions, give the intermediate 
oligoclase, labradorite, andesite, and so on. There are also the triclinic 
microchne and its monoclinic equivalent, orthoclase, both represented 
by the formula AlKSi 3 8 , and the barium feldspars, such as hyalophane, 
which may be regarded as mixtures of orthoclase with a barium salt 
analogous to anorthite in composition. Microcline and orthoclase, from 
a chemical point of view, may be considered as isomers 5 and so also 
may albite and soda-orthoclase; but the nature of the isomerism is not 
clear. It may be due to the structure of the salt as such, or to the 
existence of two isomeric acids H 4 Si 3 8 , and both possibilities are con- 
ceivable. For present purposes this problem may be left outstanding, 
and our attention can be confined to the two typical compounds, 
anorthite and albite. In the discussion, later, of the species eudidymite 
and epididymite, the question of isomeric trisilicates will come up. 
For the scapolite series Tschermak has elaborated a theory which is 
closely parallel to that of the feldspars. These tetragonal minerals 
are shown to be most easily interpretable as mixtures of two end 
compounds, meionite, Al 6 Ca 4 Si( J 25 , and marialite, Al 3 Na 4 Si 9 024Cl. 
Neither end compound has yet been found in nature quite free from the 
other; but the variations in composition, in optical character, etc., are 
all accounted for, and the theory, so far as it goes, is satisfactory. I 
have tentatively examined some possible alternative hypotheses, and 
none of them fulfills all necessary conditions so well as this scheme of 
Tschermak's. 
Upon studying the feldspars and scapolites more closely, certain 
analogies appear other than those indicated by the parallelism of the 
two series. Both groups of minerals are easily alterable, and both yield 
kaolin as a final product of the change. Furthermore, both alter into 
