WHAT'S IMPORTANT IN A ROCK NAME: INSIGHTS FROM DEVELOPING A COMMON MAP LEGEND

Naming a rock might at first seem to be straightforward.  After all, a term such as granodiorite is well-defined, worldwide.  However, when it comes to naming a rock in a geologic map legend, the task is more complex.  For example, there may be 2 granodiorite bodies in a map area, one with a gneissic fabric and an Archean age, another massive, coarse grained, and Proterozoic in age.  I can show the age difference simply by map colour, but what other key descriptive terms should be included in the description, especially if one is trying to create a common map legend for a large geographic area?  The problem becomes even more acute when naming metamorphic, altered, and/or deformed rocks (a.k.a. composite-origin rocks).  For example, is composition critical to the name (e.g., marble); or texture (e.g., schist); or protolith (e.g., metabasalt); or mineralogy (e.g., diopside marble); or a combination of factors (e.g., kyanite metapelite)?  Unfortunately, all recent rock nomenclature schemes for composite-origin rocks have focused on naming such rocks at the hand sample level, not at a macroscopic scale.  To address this problem, in the process of developing a common map legend for Ontario Geological Survey maps, a hierarchical scheme, is proposed, as outlined in brief in the table below.

Type	Subtype	Root Name (examples)	Optional Modifier (textural, mineralogical, descriptive)
Metamorphic	Structural	schist, gneiss, granolite	kyanite schist
	Compositional	marble, amphibolite, pelite	calcitic marble
	Textural	breccia, agmatite
	Hybrid	migmatite
Alteration	Hydrothermal	silicification
	Metasomatic	fenite, skarn
Structural	Brittle	cataclastite and related rocks	fault gouge
	Ductile	mylonite and related rocks
	Impact	melt, breccia