198 EDSON S. BASTIN 
The same result is brought about by the hydrothermal alteration 
of monzonite porphyry in the Clifton-Morenci district as described 
by Lindgren,’ and Dr. A. C. Spencer has shown the writer unpub- 
lished analyses exhibiting similar changes as a result of hydro- 
thermal metamorphism of porphyry in the Ely district, Nevada. 
It is evident therefore that the chemical criteria must be applied 
only to those foliated rocks which the microscope shows have not 
been subsequently subjected to weathering or to igneous or hydro- 
thermal metamorphism, in other words, to fresh foliated rocks. 
In those rare instances where igneous or hydrothermal meta- 
morphism has affected the rocks previous to the development of 
foliation there should usually be some evidence of it in the presence 
of mineralization in associated schists. 
An instance of the successful application of chemical criteria 
as an aid to determining the origin of a schist in a mining district 
is afforded by analysis III of Table II, hitherto unpublished, which 
-I have been permitted to use through the courtesy of Dr. F. L. 
Ransome. This schist consists predominantly of quartz and 
sericite with subordinate chlorite, biotite, andalusite, magnetite, 
and zircon. It forms part of the Pinal schist of the Ray district, 
Arizona, which is shown by bands of pebbles, arkosic and quartzitic 
phases to be for the most part of sedimentary origin. This schist 
is older than the mineralization, and microscopic study indicates 
that the specimen analyzed has suffered no important mineral 
changes since the development of the foliation. It will be noted 
that there is about twice as much magnesia as lime in this rock 
and about three times as much potash as soda. Alumina is 
present to the extent of 10.30 per cent above that necessary to 
satisfy the lime and alkalies present. The strong development in 
this rock of these three chemical characteristics points to its 
having been subjected to the processes of weathering and assort- 
ment previous to metamorphism, in other words, indicates a 
sedimentary origin, and affords, the writer believes, a valuable 
adjunct to textural and structural evidences in interpreting the 
origin of the rock. 
1 W. Lindgren, ‘‘The Copper Deposits of the Clifton-Morenci District, Arizona,” 
Prof. Paper U.S. Geol. Surv., No. 43 (1905), p. 168. 
