For at least two generations, the depth of underground muon
experiments has been reduced to depth in ``standard rock.'' This is by
definition the overburden of the Cayuga Rock Salt Mine near Ithaca,
New York, where K. Greisen and collaborators made seminal observations
of muons at substantial depths. Ref. 1 says
only ``Most of the ground consists of shales of various types, with
average density 2.65 g/cm2 and average atomic number 11.'' Menon
and Murthy later extended the definition to include
<Z2/A> = 5.5 and
<Z/A> = 0.5.
It was thus not-quite-sodium. Lohmann further
assumed the mean excitation energy and density effect parameters were
those of calcium carbonate, with no adjustments for the slight density
difference. We use their definition for this most important material.
(Extracted from D.E. Groom, N.V. Mokhov, and S.I. Striganov, ``Muon
stopping-power and range tables, 10 MeV-100 TeV,''
Atomic Data and Nuclear Data Tables 78, 183-356 (2001).)
 P. H. Barrett, L. M. Bollinger, G. Cocconi,
Y. Eisenberg, and K. Greisen, Rev. Mod. Phys. 24, 133 (1952).
 M. G. K. Menon and P. V. Ramana Murthy, in Progress
In Elementary Particle and Cosmic Ray Physics 9
161-243 (North Holland, Amsterdam, 1967).
 W. Lohmann, R. Kopp, and R. Voss, ``Energy Loss of
Muons in the Energy Range 1-10000 GeV,'' CERN Report 85-03 (1985).