dwelling upon

She was not dwelling upon any particular train of thought.

He would lie by the hour, eager, alert, at Thornton’s feet, looking up into his face, dwelling upon it, studying it, following with keenest interest each fleeting expression, every movement or change of feature.

In recent years, I have found myself dwelling upon two major changes throughout the park.

There was no dwelling upon what the emails even showed.

“Without dynamite in the streets” is a phrase worth dwelling upon.