No. VI. 



Ohsenations on the Trap Rocks of the Co?i7iewago Hills near 

 Middletown, Dauphin County, and of the Stony Iddge near 

 Carlisle, Cumberland County, Pennsylvania. By tlie Ho- 

 norable John B. Gibson, — Read 17 th J\ov. 1820. 



On the Connewago Hills, between Elizabethtown and 

 Middletown, these rocks are found resting on the old red 

 sandstone, which extends from the North River, near New 

 York, to the Rappahannock, near Falmouth in Virginia ; and 

 which is here about ten miles broad. They exhibit nothing 

 like stratification, but constitute the summits of the hills ; 

 the sandstone preserving a common line of elevation, above 

 which all is either trap rocks, or a grey sandy mould pro- 

 duced from them by decomposition. These rocks are ba- 

 saltiform greenstone ; but they are accompanied by some 

 of the other members of the trap family, sucli as amygda- 

 loid, wacke, 65c. This basalt is of two kinds : The first is 

 of a dark iron grey colour, with a shade of blue, sometimes 

 verging on black ; its streak is an ash grey ; it is of a com- 

 pact granular structure, and is chiefly composed of feldspar 

 and augite 01 hornblende, but as I judge, after an attentive 

 examination with a pretty high magnifying power of the 

 microscope, most probably the latter: it rings when struck ; 

 but though extremely hard, gives, with difficulty, a few 

 sparks with steel, and when broken with a hammer, often 

 flies into thin pieces with siiarp edges : its fracture is rough. 



