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in which the frequency of a word or set of
quartz monzonite quartz syenite syenogranite words is inversely proportional to its fre-
monzogranite
monzogabbro
quartz diorite monzodiorite alkali feldspar granite quency rank. The IUGS names follow this
quartzolite
quartz monzodiorite
monzonite quartz gabbro quartz monzogabbro for the first 10 and then drop off precipi-
quartz anorthosite
anorthosite tously. Many names have been used only
a few times, and then only to define a
tonalite field on an IUGS diagram, a usage that
syenite granite scarcely justifies their continued presence
on classification diagrams.
diorite Nothing in our method asks that a rock
name be tied to its origin.
granodiorite modal data in databases such as Earth‐
Frost et al. (2019) state that the lack of
gabbro Chem means that “... their quantitative
classification system, in addition to being
of limited value in the field, is not likely
Figure 2. Relative frequency of occurrence of International Union of Geological Sciences quartz- to be widely applied.” This misses the
bearing plutonic rock names in the Geological Society of America Bulletin, 1890–present. Relative point. Perhaps our field methods differ,
areas of letters are proportional to frequency. “Granite” dominates, and the top 10 make up over but we attempt to estimate modes in the
98% of the names, with the remaining 14 comprising only 1.8%.
field so that we can correlate, map, and
understand the units more effectively.
possible. As noted by Frost et al. (2019), long as the numbers are there the names Most field petrologists do so, but the lack
such systems produce arrays that radiate are not important, the similarities among of such data in databases means that such
from near the P apex and, as we noted the four estimates are clear, and the esti- data are effectively discarded. Our pro-
in Figure 3 of our original paper, one mated color index is derivable. posal aims to fix that.
Yosemite unit crosses the four-way junc- The contention that one need not know
tion where the granodiorite, tonalite, what the IUGS names mean because they ACKNOWLEDGMENTS
quartz monzodiorite/quartz monzogabbro can be looked up strikes us as similar to We thank Stan Hart for encouragement and
and quartz diorite/quartz gabbro/quartz saying that one need not memorize vocabu- the Feynman quote, and Jeanette Hammann and
anorthosite fields come together. Only lary to be fluent in a language because one Matt Hudson for help with the Geological Society
confusion can come of this, as four sepa- can always just look up all the words. It is of America Bulletin backfile.
rate field estimates of modes of a rock self-evident that the use of words that must REFERENCES CITED
from near this junction could be 20,10,50; be decoded interferes with communication. Frost, B.R., Frost, C.D., Anderson, J.L., Barnes,
20,5,55; 15,10,60; and 15,5,60. In our sys- Many IUGS names are rarely used. C.G., and Wilson, M., 2019, A more informa-
tem these are all granodiorites (or tonal- Figure 2 shows the frequency of occur- tive way to name plutonic rocks—Comment
ites, as the name is secondary), but these rence of the rock names in the quartz- by Frost et al.: GSA Today, v. 29, https://
would end up with four different IUGS present part of the IUGS diamond in the doi.org/10.1130/GSATG405C.1.
names: granodiorite, tonalite, quartz Geological Society of America Bulletin, Hogan, J.P., 2019, A more informative way to
name plutonic rocks—Comment by Hogan:
monzodiorite, and quartz diorite. One 1890–2018. As we noted in our paper, GSA Today, v. 29, https://doi.org/10.1130/
could use the full IUGS names, but as rock names generally follow Zipf’s Law, GSATG406C.1.

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