This exerpt is found in Prometheus Unbound: A Lyrical Drama in Four Acts by Percy Bysshe Shelley. 

In the stanza you have identified (scene 2.3), there are several literary devices. First, personification is found in "the rushing snow" and "sun-awakened avalanche."

Personification is defined as:

...a figure of speech in which abstractions, animals, ideas, and inanimate objects are endowed with human form, character, traits, or sensibilities.

In other words, human characteristics are given to non-human things. In the examples above, snow can move swiftly, but people rush: the intent to move with speed is implied; and, an avalanche cannot wake up.

Another literary device used is repetition: in this case, two words are repeated in both examples, and also the structure of the phrasing is repeated. The examples are "Flake after flake" and "thought by thought." 

Repetition is defined as the "return of a word, phrase, etc.," and...

Repetition is an effective literary device that may bring comfort, suggest order, or add special meaning to a piece of literature.

Repetition here is used to compare the piling of thoughts, like the piling of snow flakes.

The images of flakes and thoughts are also a part of an extended metaphor.

An extended metaphor is a comparison between to dissimilar things as if they are the same...however, the extension is present as the comparison is expanded throughout several lines. The basic metaphor is found in the lines:

Hark! The rushing snow!


The sun-awakened avalanche! whose mass


Thrice sifted by the storm, and gathered there


Flake after flake, in heaven-defying minds


As thought by thought is piles, till some great truth


Is loosened, and the nations echo round,


Shaken to their root...

The metaphor compares the enormous snow from the storm to thoughts that build up until a "great truth" is "loosened," just as snow loosens with an avalanche. The avalanche creates an echo, and "shaken to their root" refers to the after shocks when an avalanche subsides.

To delve further, this metaphor speaks to the idea of revolution: a "mob" gathered together; the "loosening" of "constraints" that have until now controlled the civilized behavior of men. The "nations echo around" reflects other nations of the world who respond to the outrageous behaviors of a "despot" (or a tyrant).

...the avalanche...is an image of revolution...

So Shelley is alluding in his poem to the idea of revolution. We might surmise he is writing about the French Revolution. As a second-generation poet, Shelley (and his contemporaries) advocated personal freedom in their poetic works; they were staunch supporters of revolutions (e.g., the French and American Revolutions).

An allusion is defined as:

…a reference, usually brief, often casual, occasionally indirect, to a person , event, or condition thought to be familiar…to the reader.

Other devices include alliteration: the repetition of a similar sound in words groups together:

...sifted by the storm...

The repetition is found in the same sound of "S."

Additional Source:

http://books.google.com/books?id=euTuaVefJvcC&pg=PA4&lpg=PA4&dq=shelley,+Thrice+sifted+by+the+storm,+and+gathered+there%E2%80%A8&source=bl&ots=1vwAnoQETu&sig=r9SW0ayu47oKy-TgB5S69iPtNoM&hl=en&sa=X&ei=1_G-T9-wCum66AG4yvzECg&ved=0CDYQ6AEwAQ#v=onepage&q=shelley%2C%20Thrice%20sifted%20by%20the%20storm%2C%20and%20gathered%20there%E2%80%A8&f=false