Abstract
A written communication on age determination was received from N. J. Snelling, and J. W. Stewart.
A considerable number of age determinations are now available from Peru. Most are by the K: Ar method on biotites and hornblendes Interpretation of the data is made difficult by the fact that granitic rocks have been emplaced at different times throughout the Phanerozoic, and extensive resetting of the K: Ar clocks in the older rocks may have occurred. The following are some of the main points which have emerged from our investigation and from the unpublished work of Dr. J. Evernden of the University of California at Berkelsy.
1. Two samples have yielded Precambrian ages (circa 660 m.y.). The analysed biotites are from gneisses within the Coastal Batholith—Arequipa Batholith intrusive belt south of latitude 15°S.
2. Five Palaeozoic ages ranging from 447 to 251 m.y. are known. They occur in the Coastal and Eastern Cordillera. Stratigraphic control is sufficient in one case to confirm an intrusion in Late Ordovician or Early Silurian and in another case Late Permian.
3. Triassic and Jurassic ages are known from the Coastal Cordillera, the Western Cordillera, and the Eastern Cordillera. The main period of intrusion appears to have occurred during the Late Triassic or Early Jurassic.
4. The Coastal Batholith yields a whole spectrum of ages from circa 100 to 10 m.y. Fewer ages are available from the Arequipa Batholith and they show a more limited range from 3
