The silver content is given as a "fineness" on the coin's reverse side. It should show something like ".999" which would mean 99.9% pure silver.
Mintmarks are small letters that are located on the reverse of the coin, just above the DO in DOLLAR. For 1878 they can only be an "S" or "CC" mintmark.
An 1847 seated silver dollar = =
They are the letters D or S on the reverse between the word ONE and the tip of the wing feathers.
Like an eagle
That depends on what silver dollar you're talking about. See the related links below for pictures of different US silver dollars.
1879 would make it a "Morgan" dollar.
A Morgan silver dollar is a large dollar coin, on the front there should be a depiction of Lady Liberty's face wearing a small crown saying "LIBERTY" on it the reverse features an eagle with olive branches and arrows and a wreath at the bottom. They were dated 1878 to 1904 and also made in 1921. The only other coin that could be easily confused with a Morgan dollar is a Peace Dollar of which some were also minted in 1921, they have the same size, but the liberty on a Peace dollar looks like there are rays coming out of her head and the reverse of a Peace dollar has an eagle perched on a rock engraved PEACE.
1836 U.S. Silver Dollar type this into your search box and click on images to see one.
The moon was a silver dollar is definitely a metaphor. Remember that for something to be a simile it must be preceded by like or as.
No such thing.
That depends. If it is a regular dollar made for circulation it contains no silver and is worth face value. But if it's a collectors silver dollar in witch case it should say something like .999 silver then its worth its weight in silver, value changes with the silver market.