The Pittsburg Coal Bed. — White. 49 
.f 
result is because, in most of the plagioclases, the zones parallel 
to lines contained in 010 reach their maximum angle of ex- 
tinction in the neighborhood of that section which serves ap- 
proximately as the principal plane of elasticity, excepting in 
anorthite. In the same manner, for the trace of 001, re- 
ferred to the trace 010, in the same zones, the angles pass a 
maximum in the neighborhood of 010. Their sum, or their 
difference, ought, therefore, to vary but little, and 010 is hence 
well chosen from all points of view. 
THE PITTSBURG COAL BED.* 
By I. C. White, Morgantown, W. Va. 
Among the rich mineral deposits of the great Appalachian 
field, the Pittsburg coal bed stands preeminent. Other coal 
beds may cover a wider area, or extend with greater persist- 
ence, but none surpass the Pittsburg seam in economic im- 
portance and value. It was well named by Rogers (H. D.) 
and his able assistants of the First Geological Survey of Penn- 
sylvania, in honor of the city to whose industrial growth and 
supremacy it has contributed so much. Whether or not the 
prophetic eye of that able geologist ever comprehended fully 
the part which this coal bed was to play in the future history 
of the city which gave it a name we do not know; but certain 
it is that the seven feet of fossil fuel which in Rogers' time 
circled in a long black band around the hills, and overlooking 
the site of Pittsburg from an elevation of 400 feet above the 
waters of the Allegheny and Monongahela, extended up the 
latter stream in an unbroken sheet for a distance of 200 miles, 
has been the most potent factor in that wonderful modern 
growth which has made the Pittsburg district the manu- 
facturing centre of America, and which bids fair to continue 
until it shall surpass every other district in the world, even 
if it does not now hold such primacy. 
That this claim for Pittsburg's supremacy is valid can 
hardly be doubted when we see its iron, steel, glass and other 
products going to every part of the western continent and 
*Vice-president's address before Section E (Geology and Geog- 
raphy), Am. Assoc. Adv. Sci. 1897. 
