It's a bit hard to interpret given the information is via song, but she's able to get a map of the mountain, including the varied composition. This mountain probably didn't form naturally, is one notable part.
Also: there's a lot of carefully, meticulously carved grand tunnels, decorated in geometric designs and only somewhat defaced by the orcs, which Brisingr should be able to fit through. The grand tunnels have stood for millennia, the song sings, through war and dragon fire and earthquake, and they were carved by those who love the stone, and they will stand for millennia yet, until the last dwarf dies. There's also a ton of smaller, newer, and more poorly thought out tunnels weaving through the mountain, which are far less stubborn about having an aesthetically continued existence. Many of them are already blocked off by assorted cave-ins over the millennia.