In Chapter Eight, what do Armstrong and Blore discuss as they finish their search for a cave?

The men discuss how someone might have poisoned Marston, and Lombard suggests that Marston had kept his drinking glass close to an open window. While everyone was distracted, someone could have reached in and put poison in his drink. Blore thinks that when they discover U.N. Owen, they will probably face a dangerous lunatic. Armstrong tells Blore that he may be wrong, and that “many homicidal lunatics are very quiet, unassuming people. Delightful fellows.”