The national era. [volume] (Washington [D.C.]) 1847-1860, 1851-03-27
AGRICULTURAL GEOLOGY. No. II.
ny JOSIAH HoLBRooK.

Granite, hornblend, and lime formations consti-
and rocky masses upon our globe. The highest
peaks OF mountains aud the lowest depths Of exca-
vations yet witnessed are principally granite for
nations Early upheavals Of the earth in its
geological history, are supposed to have given To
this rock-formed when 1t was said, Z" Let dry land
appearn-both the highest and the lowest position
Ia the piling of mountains.
All granite formations are composed essentially
of three ngredients--quartz, felspar, and mica,
combined by mechanical mixture rather than
chemical combination in almost every proportion
of these ingredients Quartz Is the most abun-
ant of the three ingredients and sometimes of It.
self constitutes large rocks, and even mountain
ranges Felspar, also, in some rocky Masses Is the
most abundant ingredient. When the rock is prin-
opaly felspar nterspersed with irregular lines OF
quartz, giving II somewhat the character Of He
brew letters it is called graphic granite, from the
Greek word eTaphO, to write. In graphic granite
the mica is entirely, or nearly, wanting being COM
posed almost wholly Of felspar, with a slight
sprinkliug Of quartz Such felspar rocks fre-
quently decompose by the action Of air and water,
forming porcelain clay, called by the Chinese haol
A large deposit Of felspar, Of a good quality for
chnaware also for porcelain teeth has been opened
and worked to some extent in Wilmington, Della
ware That deposit, also one in IIsddsm, Connect.
cut, have furnished large quantities Of felspar for
porcelain establishments In this country, in add-
tiol to considerable quantities exported TO other
countries for the same use.
A portion Of granite formations are stratiied
having slaty structure. Another portion is I'M
strstifed--the three ingredients especially the
mica, being thrown in every imaginable position
Those strati6ed are called gueiss and mica slate
The distinction between gneiss and mica slate is
the absence Of felspar In the latter the rock being
composed Of quartz and mica Of a homogeneous
structure, with smooth but frequently an uudua-
ting surface II Is more friible, and less durable
for footwalks, bridges, floors, and other purposes
exposing II to friction, than gneiss II Is also less
readily split into slabs Of large surface, and Is
hence far the most limited Of the two rocks in
their application TO purposes Of architecture
Experments-A teacher in Philadelphia once
said to his pupils, Boys, all who have their les
sons to-day at eleven O'clock may 70 with me on
geological excursion Every boy had his les
son thoroughly ut the hour named - the first
thorough lesson ever got by several Of his pupils
Similar experiments continued changed his worst
scholars into bis best
Ia one OF the New York Public Schools the
teacher was greatly annoyed by several trusnt
boys, drawn TO the docks Of the city by the attrac
tions upon the wharves. He ut length offered TO
the punctual scholars exercises in drawing, also
an opportunity TO form cabinets Of geology for the
school, their homes, and sending abroad. His In
corrigible truants Immediately became bis most
punctusl scholars and the very worst boy In school
was soon known ~ an artist, and, a. such, invited
BY clergyman Of the city 70 become the associate