This is possibly what Twain was referring to as Pentilicon

Pentéli or Pendeli, (Greek: Πεντέλη, ancient forms: Pentele, Pentelikon or Pentelicus (Πεντελικός), and Vrilissos or Vrilittos (Greek: Βριλησσός, Βριληττός), Mendeli in medieval times) is a tall mountain and mountain range situated northeast of Athens and southwest of Marathon. Its elevation is 1,109 m. The mountain is covered in large part with forest (about 60 or 70%), and can be seen from southern Athens (Attica), the Pedia plain, Parnitha, and the southern part of the northern suburbs of Athens. Houses surround the mountain, especially in Vrilissia, Penteli, Ekali, Dionyssos and north of Gerakas.
https://en.wikipedia.org/wiki/Penteliko_Mountain