According to petrarch and his contemporaries, the essential qualification for a writer was an appreciation of authors of which culture?

According to Petrarch, the essential qualification for a writer was an appreciation of classical Greek and Roman authors, such as Cicero.  Petrarch is considered by many to be the father of the Italian Renaissance.  The interest in classical civilizations and Greco-Roman culture reignited a passion for learning and the arts after the Middle Ages.  The significance that Petrarch  placed on the individual in literature, art, music, poetry, and rhetoric became known as humanism.  Today, these subjects are the basis for a classical education in the humanities.   

The one that most represents the earths mantle would be the skin since the mantle is the upper part of the earths depths which is just like the skin of an apple.