Chemical titans DuPont and Dow Chemical Co. have agreed to combine in an all-stock merger valued at $130 billion in a first step toward breaking up into three businesses, a move that pleased activist investors and could trigger more consolidation.

The "deal of three centuries," as Wells Fargo analyst Frank Mitsch dubbed it, combines two of the biggest and oldest U.S. chemical producers and will generate cost and tax savings.

Dow and DuPont shares fell on Friday after spiking earlier in the week following reports of negotiations.