After entering the upper area from finding Dr. Zemph's Journal, the secret is on the catwalk (the one partially crumbled now), left on the "T", but the button to open it is in the crypt room next to where you entered the level after finding the journal. You have to be quick, and after hitting the stone button, run! Take note of the shapes on the wall behind the treasure you find, they are the key to your next secret.
