STATE GEOLOGIST. 57 
SHIN CaM yen renee Ne ae SILLS es 64.68 per cent. 
JENIMENAMIBGIE ING > or RM Is Cale: oe DM ee ho sloAgw pets cent 
PO LASTS ren eN SS ci na i Netseh Se Se 16.89 per cent 
being an alkali-alumina silicate. 
Kaolin, on the other hand, has the formula Al,O,, 2SiO,, 2H, O and 
the percentage composition : 
Silica eects tech eg ND GU Gt hela 46.3 per cent 
EN Lettie ee iis eh Aare SDE RAIS! o 39.8 per cent 
NIN CGN NS ACA Enso eared Hn SC 13.9 per cent 
which is a hydrous aluminum silicate. 
The significant changes noticed are the removal of the potash and a 
part of the silica and the introduction of chemical water. The obvious 
physical change is in the metamorphosis from a hard, crystallized sub- | 
stance to a material soft and plastic in character. 
While the feldspar has undergone decomposition, the augite perhaps — 
has .also been attacked in a similar manner, while the mica is much 
slower to succumb to the destructive agencies and the quartz remains 
practically unchanged as far as chemical atmospheric influences go. It 
can be readily seen that thus the end product of the decomposition of 
the granite is not, in the nature of the case, a homogeneous product, but 
must consist of kaolinite clay substance, mixed with undecomposed frag- 
ments of feldspar, weathered and unweathered augite, mica and quartz. 
The material resulting from the decompositicn of the augite closely 
resembles clay substance, but instead of being white in color it is discol- 
ored by the iron from the original mineral, resulting in a red clayey mass. 
Since, however, the number and variety of aluminum silicates which 
on decomposition result in some modification of the clay substance is very 
large, it is evident that the end product must be extremely variable in 
composition, resulting in hydrous aluminum silicates of varying chemical 
and physical character, all of them being contaminated with quartz. For 
instance, in speaking of feldspar, the term includes both the acid feldspars, 
like orthoclase, albite, and oligoclase, and the basic ones, like labradorite 
and anorthite ; the iron carrying minerals may be augite, mica, hornblende, 
or even magnetite, limonite and pyrites. 
The character of the clay must invariably be dependent on the char- 
acter of the rock from which it is derived, and since there are so many 
kinds of igneous rocks, granite, syenite, dolerite, trachyte, gabbro, diabase, 
andesite, porphyry, we see again how heterogeneous a product the primary 
clay materials must be. White clays are, hence, exceptional, owing to the 
great abundance of iron carrying minerals, and are found only under 
favorable circumstances and hence are comparatively valuable. 
The clays so far discussed are primary clays, “produced by 
decomposition from the original rock 7m sitw without transporta- 
tion of the decomposed material. Since water, that all-powerful 
