198 E. B, Andrews on a Seam of Coal. 
in my geological rambles among the hills and mountains, I have 
often taken my points of compass from the coal, as I have found 
the seams exposed in the various hillsides. ( 
n the Bear Creek coal the principal planes lie in the di- 
rection S. 80° E. Besides these principal or primary planes 
planes are parallel with 
each other, and constitute 
at the point where exhib- 
ited a perfect and beautiful - 
system, and as distinctly 
marked as the primary sys- 
tem. The accompanying 
figure, (fig. 8,) shows the systems of planes in a block of the Bear 
Creek coal. The lines marked a represent the primary planes; 
those marked 0 the secondary ones. 
In very rare instances the plane b curves into the plane a, but 
generally there is a sharply defined angle of intersection. These 
angles vary in different localities in the same mine; but wherever 
the secondary planes appear, they are parallel and true to their 
own local system. The highest angle I have measured is over 
30°, but generally the angles are less. So far as I have exam- 
imed the Bear Creek coal, the secondary planes cross the pr- 
mary ones in the direction of south-of-west and north-of-east. 
This brings the primary planes on one side of the east and west 
line, and the secondary planes on the other (see fig. 4). If these 
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may be called crystalline planes, as their regularity and polish 
_ would indicate, they were doubtless formed at the time of the 
solidification of the coal, and their direction determined by ter 
restrial electricity. The great east and west currents of thermal 
tricity might induce a sort of polarity in the particles of 
