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Selected occurrences of Mn-rich urban varnish identified to date are listed in Table 1. These are divided into two categories: probable, based only on visual appearance; and confirmed, based on XRF detection of elevated Mn levels. In the second category those sites measured by pXRF are indicated in plain font. Asterisk indicate cases where the varnish was sampled and measured using laboratory XRF instruments.

Table 1: Locations with urban rock varnish. * Analyzed by laboratory XRF

Probable	Confirmed by XRF
St. Matthews Cathedral (1895), Washington, DC	Smithsonian Castle (1847–55), Washington, DC
Oak Hill Cemetery Gateposts (1865), Washington, DC	Bethesda Fountain Plaza (1864), New York, NY
Phillips Collection (c. 1900), Washington, DC	St. James Church (1884), New York, NY
Basilica of St. Peter & St. Paul (1864), Philadelphia, PA	Albany City Hall (1880-83), Albany, NY
Old Bennet School (1908–1909), Manassas, VA	Albany Academy Building (1815), Albany, NY
Crown Cork & Seal building (1904), Baltimore, MD	Salem Street Church (1871–73), Springfield, MA
Austin Hall (1881–84), Cambridge, MA	City Hall, East Longmeadow, MA
B. & A. Railroad Station (1883–85), Framingham, MA	James Hill House (1891), St. Paul, MN*

At this time, insufficient data points preclude statistical analyses, but some preliminary observations 165can be made. It is evident that urban varnish is a widespread phenomenon and not just a local Washington problem. Second, on structures built with more than one type of stone, the varnish occurs only on the Triassic sandstone. This suggests that some property of this stone encourages the growth of the varnish. An exception to this rule is the identification of varnish on the Carboniferous sandstone of the Central Park’s Bethesda Fountain Plaza in New York City. Finally, the occurrence of varnish appeared to be an anomaly on the James Hill House in St. Paul, Minnesota, since it is located far from the Newark Supergroup region. However, records show that the building stone was shipped by railroad to St. Paul from Triassic red sandstone quarries at East Longmeadow, Massachusetts, which is within the Newark Supergroup.