G. W. Tyrrell—Petrographic Nomenclature. 501 
Iddings teschenite is treated as a plutonic rock, but is regarded as 
an analcitic variety of theralite, in which the analcite “may be 
altered nepheline”’.! This view is a survival from the period in 
which the analcite in this and in other rocks was thought to be of 
secondary origin. Nepheline does occur in some teschenites, and is 
often altered ; but it does not alter into analcite. Analcite has been 
shown to be a primary constituent of teschenite, and many other 
analcitic igneous rocks ; but owing to its aqueous constitution, and 
the consequent mobility and chemical activity of its solutions or 
magma, it is an extremely efficient agent of paulopost alteration 
in previously formed mineral constituents, with the result that 
analcite itself has been regarded as secondary. In other words 
the analcite is not due to the alteration of the other minerals, but 
the alteration is due to the analcite. If analcite be recognized as 
a primary mineral constituent in teschenite, this rock is as valid 
a rock species as theralite, and deserves equal recognition in a system. 
We have no space in which to discuss the nomenclature of 
sedimentary and metamorphic rocks at any length. One point that 
struck the writer in reading Dr. Holmes’ N omenclature of Petrology 
was that the term psammnutic was treated as exactly synonymous 
with arenaceous, also pelitic with argillaceous (p. 35), and rudaceous 
with psephitic (p. 202). It would appear consonant with current use, 
however, to restrict the terms of Latin derivation to sedimentary, 
and those of Greek derivation to metamorphic rocks. We do not 
speak of arenaceous gneiss but of psammitic gneiss ; pelitic schist, 
in like manner, is regarded as a more felicitous term than argillaceous 
schist ; whereas the adjective psephitic usefully describes the 
metamorphic rocks derived from the metamorphism of con- 
glomerates, etc. The term arenaceous conveys (at least to the writer), 
a suggestion of past, present, or potential looseness of grain, 
whereas the corresponding less well-known Greek term does not 
carry this suggestion. Similarly argillaceous conveys a suggestion 
of softness, plasticity, and extremely fine grain, which is not so 
directly conveyed by pelitic except: perhaps to Greek scholars. 
Hence it is suggested that the terms psammitic and pelitic might 
usefully be restricted to the hard metamorphic rocks which have 
been changed beyond the limits implied by the corresponding 
Latin terms. 
Grabau’s term rudaceous, applied to coarse detritus such as gravel, 
shingle, talus, and their compacted representatives, completes the 
trilogy of Latin adjectives for the principal gradings of sedimentary 
rocks. The term psephitic is not quite synonymous with rudaceous 
as it also conveys the suggestion of rounded pebbles. This sense 
is also seen in the use of the term psephicity by Mackie to indicate 
degrees of roundness in sand grains. But as during metamorphism 
angular fragments tend to become more or less rounded, the term 
psephitic will apply with considerable degree of exactitude to those 
1 Igneous Rocks, vot. ii, 1913, p. 248. 
