The stock market crash of 1929 vaporized prospects for promotional profits, and slack demands for output during the Great Depression curtailed mergers based on economies of scale. Figure 3 shows that the merger rate fell drastically and remained low until the third big wave appeared late in the 1950s, again, paradoxically, on the heels of major antitrust legislation---the Celler-Kefauver Antimerger Act of 1950. This third wave lasted into the early 1970s. The bulk of the mergers during this period were conglomerate mergers, which combine firms from unrelated industries. For example, DuPont's (a chemical company) 1982 acquisition of Conoco Oil was a conglomerate merger.
(BBC News 2008 Oct: GM and Chrysler in Merger Talks)