fa} NEW YORK STATE MUSEUM 
Such an interprétation is supported also by the fact that older 
formations were certainly available to furnish the supply. The 
bluestone is of Devonian age, and beneath strata of that age and 
beneath an unconformity lies the so-called Hudson River series, 
which is made up wholly of slates and graywackes and closely 
related varieties of sediments. | 
As far as the Devonian bluestone itself is concerned, the story 
could end here; but the similarity of the graywackes of Hudson 
River age to this bluestone of much later origin challenges an exam- 
ination of it also by the same methods. When this is done it is dis- 
covered that the older graywacke of the Hudson River series is 
made up of original aggregate grains also. In other words, it is 
made up, in like manner, chiefly of complex aggregate grains, most 
of which must have been derived from a still older fine-grained rock 
series not very unlike itself. 
But when one looks below the next unconformity for a corre- 
sponding rock series, a real difficulty is encountered. Apparently 
the next underlying series is the ancient crystalline gneisses and 
schists of the Highlands of the Lower Hudson, and no rocks similar 
to the grains in the graywacke occur at all. 
On the south side of the Highlands, however, there is a crystalline 
series of disputed age, which was originally, before its extensive 
metamorphism, or in those parts not so completely metamorphosed, 
very like the required quality of material. If, therefore, this crystal- 
line series, the so-called Manhattan schist-Inwood limestone-Lowerre 
quartzite series, is really older than the Hudson River slate-Wap- 
pinger limestone-Poughquag quartzite series, it could furnish the 
required material. Nothing else is known that could do it. 
In so far, therefore, as this line of reasoning goes, it supports 
the contention that these two series of rocks are really of very dif- 
ferent age and not one the metamorphosed representative of the 
other, as sometimes thought. Thus interpretation petrography may 
throw real light on a problem which is quite outside of the usual 
field covered by petrographic description. 
4 It is usually considered comparatively easy to distinguish 
between a quartzite and a limestone, yet even this may become a 
very difficult thing to do. The writer has had occasion to work with 
material of this kind, in which a microscopic or a chemical deter- 
mination of the relative amounts of silica would not suffice. The 
only method left was that of petrographic interpretation on the basis 
of origin and subsequent modification, including contact metamor- 
