US silver coinage was typically 90% (.900 fine) silver, 10% (.100 fine) copper. Less than sterling (.925 silver/.075 copper), but greater than many European compositions of the day (.800 silver).
It's important to remember that in the late 19th century, the US was glutted with silver from the western mines, and loads of it was propped up by the US (hence all the Morgan Dollars that never circulated, among others). But that's another story (and for another board, no doubt!).
Here is a link to one of several previous discussions on the subject of coin silver. Officially at present, I believe the purity of coin silver is 90%, but it hasn't always been that way. You might want to check out several of the previous threads which covered this subject.