A Bang-Bang Birthday

The bull market celebrates No. 5 with a new rampage

As parents know, a birthday party for a five-year-old is usually marked by wild noise, frantic rushing about and manic exuberance. Even so, there have been few such events as boisterous as the one on Wall Street last week. Growing at a pace that once would have seemed impossible, the bull market that was born five years ago last Thursday showed off its muscles by rampaging past one historic high in stock prices after another.

It was a week-long stampede of amazing force. The time required for the Dow Jones industrial average to get from one 100-point mark to the next...