In order to understand the physical properties, composition, age and growth processes Of the lower continental crust, it is necessary to obtain samples that have resided at deep crustal levels for substantial periods of time. Rocks that equilibrated at lower crustal temperatures and pressures occur either in large tracts of surface outcrop (high-pressure granulite facies terrains, see Mezger, this volume; Percival et al., this volume) or occur as accidental fragments (enoliths) that are carried rapidly to the Earth's surface by fast-erupting volcanics. Major differences exist between these two types of lower crustal rocks. First, if one considers the age of crust in which the two types of lower crustal samples occur (Fig. 7-1), it is apparent that many granulite facies terrains are Archean whereas very few granulite xenolith localities are situated in Archean crust. Of the high-pressure terrains shown in Figure 7-1a, 51% are Archean, 32 …