164 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

layers of muscovite. In the micaceous layers are numerous broken
crystals of tourmaline whose fragments are separated along a single
line as though they had been compressed and pulled apart by some
earth movement.

THE MARBLE. —The marbles of the Piedmont Plateau are of in-
terest topographically, structurally, mineralogically and economic-
ally. There are few areas in Maryland where the dependence of topo-
graphy upon the nature of the underlying rocks is better shown than
in the contrasts between the flat, narrow valleys in the marbles and the
abrupt ridges or gorges of adjacent gneisses and quartz-schist.
Geologically the marbles are younger than the gneisses and quartz-
schist, but the detailed relations are obscured by great structural com-
plexity and recrystallization. The general lines of structure might be
inferred from the areal distribution were this not so anomalous and
irregular as to render any explanation unsatisfactory. It is clear,
however, that it is to be accounted for not by folding alone, but by
folding accompanied by thrusts and faults at several successive periods.

The marbles of this division differ in texture and composition from
the finer and more compact crystalline limestones of the western or
semi-crystalline area. In the latter the impurities are in the form of
thin, argillaceous bands, while in the former they are represented by
layers of accessory minerals, including tremolite, white pyroxene, green
muscovite, brown and black tourmaline, scapolite, quartz, pyrite and
rutile, which correspond more or less closely with the original bed-
ding planes. The marbles are often dolomites, frequently showing
over 40 per cent of magnesium carbonate. The Baltimore county
marbles are extensively quarried, either for burning or for use as a
flux, or as a building stone (magnesian).

THE GABBRO. —One of the most ancient and most extensive of the
three eruptive rocks which so abundantly intrude the gneiss complex
is the gabbro. There are three main areas of this rock within the
limits of the state—the Stony Forest area of Harford and Cecil count-
ies; - the great belt or sheet which extends from the north of Conowingo,
on the Susquehanna river, in a south-southwest direction to Baltimore
city; and the irregular intrusive area which is mainly developed to the