Page 42 - i1052-5173-29-6

P. 42

COMMENTS & REPLY

A More Informative Way to Name Plutonic Rocks—Reply

Allen F. Glazner, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA; John M.
Bartley, Dept. of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA; and Drew S. Coleman, Dept. of
Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA

I learned very early the difference between The IUGS term “granite” applies to any by saying that it is night. We contend that
knowing the name of something and composition within the shaded pyramid if the color index is observed (as it must be
knowing something. —Richard Feynman (Fig. 1) defined by the inequalities and to apply an IUGS name), then it should be
We appreciate that our paper has gener- equation above. We know by experience reported and not discarded.
ated comments and thank their authors that real granites lie somewhere near the We disagree with Hogan (2019) that
for giving us the opportunity to clarify base of this volume, but the IUGS name, rock classification is no different from
some of the points that we made. even with a modifier such as “leucocratic,” biologic classification. A biologist keying
We assume that anyone who uses the gives little help. In contrast, Figure 1 out dogs and cats will find a split after
International Union of Geological shows 500 variations of a 30,30,30 granite Order Carnivora with dogs at the end of
Sciences (IUGS) classification to name a wherein normally distributed numbers one branch, cats at another, bears at
rock has estimated modal data, whether with a mean of five were added to the another, and so on; there are no doggish
by eye in the field, by point counting, by modal abundances. Even with such varia- cats or cattish bears—they are discrete
electron-beam methods, etc. The main tion in the estimates, the composition of species owing to discrete genomes, which
point of our paper was simply that these the rock is narrowed down far better than is why the Linnaean system has served
data should be part of the name rather with the bare IUGS name. biologists so well. Not so with igneous
than discarded or left in a field notebook. Hogan (2019) states that the IUGS clas- rocks; even the volcanic and plutonic
Our system permits the use of the IUGS sification is quantitative. It is, at the same realms grade into one another. Thus, any
name if one wishes. However, adding level that knowing a postal code narrows system with sharp boundaries, no matter
modal data to the name permits current down where someone lives—not very how well-intentioned, will split continua
terminology to be simplified, and it per- precisely. Similarly, he contends that color of rock compositions.
mits name boundaries to be fuzzy without index is given quantitatively by words such The IUGS system almost seems to have
loss of precision. In our view, fuzzy name as “leucocratic,” which again are quite been designed to carve up cogenetic calc-
boundaries have at least two advantages: imprecise; this is akin to noting the time alkaline suites into as many boxes as
the names better depict the nature of
modal variation, and they eliminate the Figure 1. QAPM tetrahedron and QAP (quartz, alkali feld-
use of multiple rock names to refer to spar, plagioclase) triangle showing 500 random varia- Q
suites of closely similar rocks. tions on a 30,30,30 granite. The shaded pyramid and
quadrilateral are the International Union of Geological
As an example, calling a rock a 30,30,30 Sciences granite fields. Naming a rock “granite” only
granite tells you rather directly what is in places it somewhere in the shaded pyramid, whereas
calling it a 30,30,30 granite places it in the center of the
the rock. In contrast, calling a rock “gran- QAP triangle and 10% off the base.
ite” is vague; the IUGS name “granite” can
only be quantified as two inequalities plus M
an equation, in four unknowns:

02. q 06. (1)
qa p

01. p 065. (2)
a p
A P

qa pm 100 (3)
where the variables are modal abundances Q
of quartz (q), alkali feldspar (a), plagioclase
(p), and the sum of everything else (m). A P

42 GSA Today | June 2019

37 38 39 40 41 42 43 44 45 46 47