First identified in 1969, komatiites are Earth's oldest known volcanic rocks and contain three times as much magnesium as do most volcanic rocks. This chemical composition suggests that komatiites formed from the hottest lava known ever to have erupted:a high concentration of magnesium changes the physical properties of lava so that unusually high temperatures would be required for the lava to exist as a liquid.

    Komatiites' discovery was surprising in light of then-current geological theories about magmas, molten rock that forms in the Earth's mantle (the layer beneath the crust) and composes volcanic lava eruptions. Prior to 1960, geologists Bowen and Hess disagreed over whether or not the very high temperatures needed to produce magmas rich in magnesium could have existed on Earth. Hess suggested that the presence of water, probably released from minerals decomposing in the Earth's mantle, might have meant that a high-magnesium magma could have existed at a lower temperature. But Bowen showed experimentally that the high temperatures were indeed necessary. By 1960, it was generally accepted that volcanic rocks with such high levels of magnesium could not exist, and thus the discovery of koma tiites changed geologists' assumptions about the characteristics of the Earth's mantle around the time of the formation of komatiites, between 2.5 and 4 billion years ago.

  Colonial historian David Allen's intensive study of five communities in seventeenth-century Massachusetts is a model of meticulous scholarship on the detailed microcosmic level, and is convincing up to a point. Allen suggests that much more coherence and direct continuity existed between English and colonial agricultural practices and administrative organization than other historians have suggested. However, he overstates his case with the declaration that he has proved ''the remarkable extent to which diversity in New England local institutions was directly imitative of regional differences in the mother country."
  Such an assertion ignores critical differences between seventeenth-century England and New England. First, England was overcrowded and land-hungry; New England was sparsely populated and labor-hungry. Second, England suffered the normal European rate of mortality; New England, especially in the first generation of English colonists, was virtually free from infectious diseases. Third, England had an all-embracing state church; in New England membership in a church was restricted to the elect. Fourth, a high proportion of English villagers lived under paternalistic resident squires; no such class existed in New England. By narrowing his focus to village institutions and ignoring these critical differences, which studies by Greven, Demos, and Lockridge have shown to be so important, Allen has created a somewhat distorted picture of reality.
  Allen's work is a rather extreme example of the ''country community" school of seventeenth-century English history whose intemperate excesses in removing all national issues from the history of that period have been exposed by Professor Clive Holmes. What conclusion can be drawn, for example, from Allen's discovery that Puritan clergy who had come to the colonies from East Anglia were one-third to one-half as likely to return to England by 1660 as were Puritan ministers from western and northern England? We are not told in what way, if at all, this discovery illuminates historical understanding. Studies of local history have enormously expanded our horizons, but it is a mistake for their authors to conclude that village institutions are all that mattered, simply because their functions are all that the records of village institutions reveal.