Allen and Greenough's New Latin Grammar, section 345

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Genitive of Quality.

345.

The Genitive is used to denote Quality, but only when the quality is modified by an adjective: -

vir summae virtútis, a man of the highest courage. [But not vir virtútis.]
magnae est deliberátionis, it is an affair of great deliberation.
mágni formica laboris (Hor. S. i. 1. 33),the ant [a creature] of great toil.
ille autem suí iúdicí (Nep. Att. 9), but he [a man] of independent (his own) judgement.

NOTE: Compare Ablative of Quality (§ 415) . In expressions of quality, the genitive or the ablative may often be used indifferently: as, praestantí prudentia vir, a man of surpasing wisdom; maximí animí homó, a man of the greatest courage. In classic prose, however, the genitive of quality is much less common than the ablative; it is practically confined to expressions of measure or number, to a phrase with eius, and to nouns modified by magnus, maximus, summus, or tantus. In general the Genitive is used rather of essential, the Ablative of special or incidental characteristics.

a. The genitive of quality is found in the adjective phrases eius modi, cuius modí (equivalent to tális, such; qualis, of what sort): -

eius modí sunt tempestátés cónsecútae, utí (B. G. iii. 29), such storms followed, that, etc.

b. The genitive of quality, with numerals, is used to define measures of length, depth, etc. (Genitive of Measure): -

fossa trium pedum, a trench of three feet [in depth].
múrus sedecum pedum, a wall of sixteen feet [high].

For the Genitive of Quality used to express indefinite value, see § 417.