THE 1921-22 SHALER MEMORIAL EXPEDITION TO SOUTH AFRICA: R. A. DALY'S INFLUENCE ON BUSHVELD PIONEERS G. A. F. MOLENGRAAFF AND A. L. HALL

"Since the time of the Caesars “something new” has ever been coming “out of Africa”... Not the least of the finds is the amazing body of rocks called the Bushveld Complex..." Thus wrote Harvard's erudite R. A. Daly (1871-1957) in 1928, citing Pliny the Elder. He had visited this "major geologic feature of our planet" (von Gruenewaldt, 1985) in 1922 during the Shaler Memorial Expedition, accompanied by C. Palache and F. E. Wright. Their guides had been the Dutch geologist G. A. F. Molengraaff (1860-1942) and the English-born A. L. Hall (1872-1955). Molengraaff had discovered the "world's most spectacular igneous assemblage" (Knopf, 1941) before the Anglo-Boer War cut short his 1897-99 tenure as State Geologist of the Transvaal Republic. Between 1904 and 1911 only, Hall had covered 38,180 km by foot and wagon, mapping 41,043 sqkm of a recently hostile land and tracing 16,995 km of contacts. In a classic 1932 monograph, he generously acknowledged that "the year 1922 is a landmark, for it saw the visit of the Shaler Memorial Expedition." On the resulting publications (Daly and Molengraaff, J. Geol. 32, 1-35, 1924; Daly, GSA Bull. 39, 703-768, 1928), Hall commented: "There is not a page that a student of the Bushveld can afford to pass over."

Of Daly's mostly superseded interpretations, at least two remain valid: (1) Massive "felsite" (now Rooiberg Group) is part of the Bushveld Complex. As oldest unit of the Complex, it holds vital clues to its origin. Previously placed in the overlying Waterberg Group, it had been interpreted as the intruded roof. Daly observed a post-Rooiberg pre-Waterberg unconformity. (2) The Bushveld Complex is surficial; Daly and Molengraaff called it "a colossal differentiated lava flow." No roof exists but about 1 million cukm of layered mafic rocks, granite, granophyre and "felsite", cumulatively 14 km thick, intrude each other.

Over 1,000 publications on the 2.06 Ga Bushveld Complex and its rich Pt and Cr ores have brought no consensus on evolution, structure, tectonic control, and "the bewildering array of felsites, granophyres and granites" (Wager and Brown, 1967). In 1925, Hall and Molengraaff ascribed the Bushveld Complex and the neighboring Vredefort dome to "one grand cycle of magmatism." Daly concurred until 1947. He then proposed an impact origin for Vredefort but made no further mental leap to the Bushveld.