President Grover Cleveland used federal troops to break up a labor strike, and President Eisenhower sent federal troops to desegregate Central High School in Little Rock, Arkansas. Which clause of the Constitution gave them this power?

Afine-grained igneous rock forms as a result of slow cooling.

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.