The dean of Columbia College has abruptly resigned over what she called administrative changes that would diminish or eliminate her authority, leaving the undergraduate division of Columbia University without a leader two weeks before the start of classes.

Michele M. Moody-Adams, the first female and the first black dean of the college, said in an e-mail sent Saturday to Columbia alumni and donors that she planned to stay through the academic year. But on Monday, the president of Columbia University, Lee C. Bollinger, said in a statement that it was in the best interest of the college and the university for her to step down immediately. He promised that an interim dean would be named soon.

In the Saturday e-mail, Dr. Moody-Adams wrote that the university had begun to “transform the administrative structure” of the faculty of arts and sciences, compromising her authority over “crucial policy, fund-raising and budgetary matters.” She said that she had repeatedly expressed concerns that the changes would affect “the college’s academic quality and financial health,” and that she recently realized that “the structural transformations intended to fundamentally alter decision-making in and for the college cannot be stopped.”

The resignation, which was reported over the weekend by a student news blog, Bwog.com, and Columbia’s student newspaper, The Spectator, is the second in three months by a prominent African-American administrator at the university. In June, Claude M. Steele, who was the university’s provost, left to become the dean of Stanford University’s School of Education. The departure of two senior administrators so close together is highly unusual for an elite university.