166 The American Geolngist, September, 1894 
parent and not real. The fossiliferous blue limestones and 
sandstones, dipping to the northwest lie on the upturned edges 
of the series a few hundred feet to the west. But following 
along the strike of both outcrops, the white and the blue, tin- 
two come together and completely cover the gneiss just north 
of the Trotter mine. 
Instances have been mentioned in this paper where anti- 
clinal axes, one flank being blue and the other white, at one 
end of a hill, have both flanks white at the opposite end. No 
gneissic anticlines have been observed in the valley, while 
anticlinal structure is very common in the white limestones 
and with white and blue, as mentioned. 
The statement that gneisses occur interstratified with the 
white limestone is erroneous. There is no phenomenon of the 
kind to be observed. Granite, greenstones, and scapolite dio- 
rite are of frequent occurrence, but they are doubtless erup- 
tives; the granites are certainly so. 
5. Identity in chemical composition. It has always been 
considered that while the blue limestone is magnesian, and 
therefore a dolomite, the white is purety calcareous; and this 
has been used as an argument for considering them as distinct. 
As a matter of fact, however, as the writer has shown by an 
extended series of chemical analyses,* the white limestone is 
also dolomitic, and it was further shown that the percentage 
of magnesia varies from place to place in each, and that spec- 
imens of both can be obtained which have the same compo- 
sition. 
It is true, that in the immediate contact with igneous rocks, 
the carbonate that is present is essentially that of lime, and 
that the rock often shows a decreasing content of magnesian 
carbonate as the eruptive is approached; but this ma} T be 
readily explained by a consideration of the following facts. 
The white limestones are well charged with crystallized 
minerals; the blue limestone is free from them. The transi- 
tion zones show mineralization in a decreasing degree fron the 
white to the blue. The chemical composition of the white 
and blue rocks does not vary essentially. Magnesia is not 
present in a iixed percentage in dolomitic limestone, and it is 
certainly not constant in the blue one of Sussex county. The 
*Amkkican Geologist, March, 1894. 
