They are called a literary letter or a epistle
A serpent and an elephant. It was a town of machinery and tall chimneys, out of which interminable [unending] serpents of smoke trailed themselves for ever and ever, and never got uncoiled. It had a black canal in it, and a river that ran purple with ill-smelling dye, and vast piles of building full of windows where there was a rattling and a trembling all day long, and where the piston of the steam-engine worked monotonously up and down, like the head of an elephant in a state of melancholy madness.
Answer:
A theme is the main idea or main point in a story.
