THE CONSTITUTION OF THE SILICATES. 
By F. W. Clarke. 
CHAP TEE I. 
INTRODUCTION. 
In the solid crust of the earth the silicates are by far the most 
important constituents. They form at least nine-tenths of the entire 
known mass, and comprise practically all of the rocks except the sand- 
stones, quartzites, and carbonates; even these exceptions are commonly 
derivatives of the silicates, which break up under various conditions, 
yielding simpler bodies of their own class, together with free silica and 
limestone. From a geological point of view, therefore, the silicates are 
of fundamental importance, and a study of their inner constitution may 
be reasonably expected to shed light upon many serious problems. 
For example, every primitive rock or eruptive mass contains an aggre- 
gation of silicates, each one of which is capable of undergoing chem- 
ical change in accordance with limitations imposed by the structure of 
its molecules. When these changes take place, secondary compounds, 
alteration products, are formed, and in time the rock becomes trans- 
formed into new substances, quite unlike those which originally existed. 
A knowledge of the processes which thus occur should be applicable 
to the study of the rocks, and should ultimately render it possible so 
to investigate a metamorphosed mass as to clearly indicate its origin. 
These processes are dependent upon chemical structure, and the 
study of this with regard to the silicates is the purpose of the present 
memoir. 
From the standpoint of the chemist the problem under consideration 
is one of great importance, but also of great difficulty. Some of the 
difficulty is real, some only apparent. At first sight the natural sili- 
cates appear to be compounds of great complexity; but this difficulty 
becomes much less serious after careful examination. Few of the nat- 
ural silicates exist in even an approximately pure condition; many 
that seem fresh have undergone traces of alteration ; isomorphous mix- 
tures are exceedingly common ; and much confusion is due to defective 
analyses. By multiplied observations these difficulties can be elimi- 
nated from the problem, but others yet remain to be disposed of. The 
organic chemist, to whom most of our knowledge of chemical structure 
