http://www.sacred-texts.com/nos/mrg/nosmrg02.htm

Century II Quatrain 70

The dart from the sky will make its extension,
Deaths speaking: great execution.
The stone in the tree, the proud nation restored,
Noise, human monster, purge expiation.

If we believe Nostradamus prophesied past major historical events and people accurately, and as recently as the Kennedy assassination, then I don't think it is a stretch to believe that he probably received a prophecy about the events surrounding 9/11, and wrote at least one quatrain about it. Whether this particular one is about 9/11, I do not know, On the presumption that it could be, I have selected particular definitions that might best fit a 9/11 interpretation of this quatrain. They are all from merriam-webster.com.

The dart from the sky will make its extension,

dart (noun): 1 archaic : a light spear b (1): a small missile usually with a pointed shaft at one end and feathers at the other
extension (noun): 2 a: the total range over which something extends

Deaths speaking:

This could mean people died while they were talking. An example: http://youtube.com/watch?v=iK8qJ1cJ9EA

great execution.

great (adjective):1 a: notably large in size : huge
execution (noun): 5 archaic : effective or destructive action

The stone in the tree,

A stone hitting a tree and causing debris like fruit and leaves to fall could be a visual interpretation of a plane hitting a building and the consequential falling debris.

The proud nation restored,

proud (adjective): 1: feeling or showing pride: as a: having or displaying excessive self-esteem
restored (transitive verb): 3 : to bring back to or put back into a former or original state : renew

This line would mean a proud nation would be humbled and lose its pride.

Noise, human monster, purge expiation.

noise (noun): 3: common talk : rumor; especially : slander
monster (noun): 4: something monstrous; especially : a person of unnatural or extreme ugliness, deformity, wickedness, or cruelty
purge (verb): 1 a: to clear of guilt b: to free from moral or ceremonial defilement
purge (noun): 2 a: an act or instance of purging b: the removal of elements or members regarded as undesirable and especially as treacherous or disloyal
expiation (noun): 1 : the act of making atonement.

“Noise” could be the 9/11 Truth Movement. People who accept the truth might describe a president who murders his own people as a “human monster”. “Purge” and “expiation” is what will (hopefully) happen in this country after the truth is accepted by the majority.