As far as historical treasures go, this is a big one.

Found in the basement of an elderly descendant of Andrew Haswell Green -- the man President Theodore Roosevelt once dubbed "the father of greater New York" -- thousands of items, including many of national significance, are seeing the light of day for the first time in more than a century.

"Frankly, the whole thing is very overwhelming," said Bill Ralph, a freelance historian hired to catalog and research the Green finding. "I've always kind of thought of it as a time capsule of 250 years."

CLICK HERE to see some of the items from the Andrew Haswell Green auction.

From a rare copy of George Washington's will and the original letters of 18th and 19th century presidents to dozens of haute couture dresses and century-old Tiffany silver, the collection is expected to fetch more than $1 million at a four-day auction that ends Sunday.

"This is an unprecedented group of things," said auctioneer Richard Oliver, president of the Maine-based R.W. Oliver's, which is handling the auction.

Most of the items hadn't been touched since shortly after Green's murder in 1903. Boxed up and passed down from descendant to descendant, they wound up in the Kennebunk, Maine, basement of Julia Green, a distant niece.

Green, unmarried and childless, died in 2009 and left her possessions to her niece and nephew, Lisa Buchanan, 51, and John Green, 54.

"It was, 'Oh, my God,'" John Green said of the moment they realized the scope of what was actually in the hundreds of boxes. "We were dumbfounded, to say the least."

John Green, who lives in Kennebunk, and Buchanan, of Denver, Colo., said they and their aunt always knew they probably had thousands of items of significance to the family and its history, but had no idea they were in possession of anything that could be considered a national treasure.

"My aunt had kind of squirreled it all away," Buchanan said.

But once the siblings were put in charge, they decided they wanted their great-great-great uncle's collection to go to "people who had a stake in history."

"I want these things to be in the right hands, whether it's the Smithsonian or some private collector; people who appreciate this," Buchanan said. "People who are responsible."

Auctioneer Oliver said he's expecting patrons to the auction from schools across the country, as well as noted museums and private individuals.

One irony, historian Ralph noted, is that Julia Green's father, the previous keeper of the collection, wrote his daughter that he had tried unsuccessfully around 1947 to give some of the items to Yale University and and the New York Historical Society.

"He said, "I have an enormous pile of trash here that might be of interest to you.' Luckily they declined," Ralph said. "He didn't fully know what was there."

The proceeds of the auction will go to John Green and Buchanan. But because the collection is so vast and neither of them made it through ever single item, John Green said he's prepared to place a few bids himself.

He said he's interested in a few antique doorstops and a map of the Kennebunk seashore in the 1800s. His wife, he said, is interested in the Tiffany silver.

The family kept "very little," he said. "Just a few sentimental things."

Washington's will, Ralph said, was a particularly amazing find. Considered a turn of the century-type facsimile, transcribed, errors and all, from Washington as he neared his death, historians knew of only 13 in existence and all were accounted for. This copy is now the 14th.

And of the nine presidential letters given to Andrew Green from a New York judge -- from Thomas Jefferson, James Monroe and Andrew Jackson -- one was especially interesting to Ralph. Written in 1820, Jefferson had penned Monroe at the White House and asked for his help in giving political advice to a Revolutionary War officer.

"It's discussing a person who is not actually identified in the letter," he said, estimating the note could go for as much as $100,000 at the auction. "It was somebody who was evidently known to both of the gentlemen. It was somebody who was seeking public office."

Washington's will and the presidential letters will go on the auction block this evening.

Many of the items date back to the 1700s and one family land deed, an original, is dated 1631. One piece that caught Ralph's eye was a purse -- a man's purse from 1760. Hand-sewn with "William E. Green" stitched into it, the purse was a common accessory for men in those days, Ralph said, as it was considered improper for women to carry money.

It has taken almost a year to go through all the boxes and bags and catalog and research each piece. It was a daunting process, Ralph said, that uncovered between 8,000 and 10,000 items. Most of the work was done right in Julia Green's 800-square-foot basement.

"We were faced with boxes of books and papers and toys and chinaware and silver," he said. "There were large plastic bags filled with costumes from the 19th century and earlier."

The historical items don't just represent a broad swath of Americana, but the personal lives of a prominent Northeast family with roots in Worcester, Mass.

Born in 1820, Andrew Green, the son of a lawyer, was the grandson of Dr. John Green -- a major player in early Worcester history and the man who built the sprawling Green Hill mansion there in 1755.

As an adult, he made his way to New York City, where he was credited with working for five decades to help create the New York City known today.

In 1869, Green -- also a onetime city comptroller -- headed the Central Park Commission and helped to create a cultural center that led to the building of the American Museum of Natural History, the Metropolitan Museum of Art and the Bronx Zoo.

Green also was behind the decision to merge the then-separate five boroughs into one city.

On Nov. 13, 1903, as he walked up to his Park Avenue home around 1 p.m., he was accosted by a man with a gun. The man, Ralph said, had mistaken Green, even at 83, for the man the gunman believed was seeing his mistress. When Green ordered the man off his property, he was shot in the back.

After Green's death, the family decided to vacate the Green Hill mansion in Worcester and sell it to the city. They took what they wanted from a history museum the family had set up inside the house and boxed up the rest. In 1905, the boxes that would become the collection up for auction today were sent to Green's nephew in New Cannan, Conn.

The site began operating as a 482-acre park. The house was torn down in 1957.

Despite Green's contributions to the city -- 1.5 acres and $4.5 million was recently set aside in Manhattan for a park in his honor -- Oliver said he found it curious that the auction hasn't piqued more of an interest from New York.

Manhattan borough historian Michael Miscione was aware of the auction but twice declined to comment.

Even though Julia Green was extremely protective of the family belongings, Buchanan said she thinks her aunt would understand why she and her brother agreed to the auction.

She was "protective, possessive, would not give away a thing," Buchanan said. "But I think she also understood that we would be trying to be responsible."