Guilford Quartz Monzonite

Guilford Quartz Monzonite Stratigraphic range: Silurian
Photographed on a boulder at the old Guilford Quarry on Guilford Road, Columbia, Maryland.
Type	igneous
Lithology
Primary	monzonite
Location
Region	Piedmont of Maryland
Extent	central Maryland
Type section
Named by	Cloos and Broedel, 1940

The Guilford Quartz Monzonite is a Silurian or Ordovician quartz monzonite pluton in Howard County, Maryland. It is described as a biotite-muscovite-quartz monzonite which occurs as discontinuous lenticular bodies which intrude mainly through the Wissahickon Formation (gneiss).

The extent of this intrusion was originally mapped in 1940 as the "Guilford granite". It was given its current name in 1964 by C. A. Hopson. Hopson grouped the Guilford Quartz Monzonite with the Ellicott City Granodiorite and the as "Late-kinematic intrusive masses."

The Guilford Quartz Monzonite was described in 1898 as "perhaps the most attractive stone in the state" by Edward B. Mathews of the Maryland Geological Survey. He provides this detailed description of the granite:

The rock of this area differs from all of the other granites of the state in the persistent presence of both light and dark colored micas. Thus, according to the German classification, it is the only "true granite" in the state. Other granites may have muscovite as a constituent, but it is not so abundant or typical as in the present instance. Both of the micas are products of the original crystallization of the molten rock magma, and they are frequently in parallel growths. The biotite, which is especially rich in iron, possesses a very dark color, but shows no evident disintegration or decomposition. The feldspar is almost entirely microcline, which shows the cross-twinning very clearly, and appears clear and fresh with very few included flakes or small cystals. These microclines form the largest individual areas in the rock mass, sometimes reaching 0.15-0.2 of an inch (4-5 mm.) in diameter, while the clear transparent grains of quartz average less than 0.01 of an inch (.03 mm.). The individuals are interlocked in a mosaic, which indicates that the rock can well withstand any pressure to which it may normally be subjected. The mica flakes are small and evenly disseminated, so that they do not injure the polish which may be given to the rock in preparing it for monumental purposes.