Basically, thick, black clay/mud.
The decrease in depth was so small over distance, that nobody even knew they ran aground until men on the stern saw the screws churning up mud, and the condenser temperatures soared when the inlets got clogged with the goo. People walking didn't even so much as brush a bulkhead.
I am a Navy guy, but this story just has the best of everything, even a scar at the end for the ship. If there was ever a time where a Captain should be held to account for grounding a ship due to rank arrogance and stupidity, Captain Brown is that man. A textbook on how not to be a Captain.