It is guessed that the Yorkshire county miners crossed the Black and Tan English Terrier, this dog was rough-coated, and the long-coated, blue-gray Waterside Terrier breeds were infused in the Scottish Terriers.
The Maltese and Skye Terrier are also possibilities.
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The ONLY reference I have (and others have found) to the Maltese being used was a pronouncement by a "dog writer" (a reporter of sorts) of the day (cira 1880's). In one of his critques of a current dog show......he remarked as to the extreme improvement in the "silk coated Yorkshire Terrier" of the previous 20 years or so. There is mention of the reporter and his proclaimation in Joan and Janet Gordon's book...."The Yorkshire Terrier". And when I saw this in the book.....I researched and found the original article.....this "reporter" was basicly just making note of the "improved silk coat". He could not believe ordinary breeders had been able to accomplish such a feat.....they just had to use the Maltese blood. <G> I have many books from the early years of the Yorkshire Terrier....and none of the breeders of that era agree that this was done.....after all....how in the heck would they retain the saddle pattern....a Maltese does not have a saddle....it is all one color. And a Yorkie does have a unique body style.
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In 1865, the foundation sire of the Yorkshire Terrier breed, was born. Huddersfield Ben was owned by M.A. Foster, and he enjoyed a very public life, to popularize the breed in England. He demonstrated that he was very successful in the
rat killing contests (these were quite popular in the 19th century), and he won more than 70 prizes as a show dog as well. Ben was also proclaimed and widely known as the "best stud in England" in that era. He sired many wonderful Yorkies during his lifetime....short as it was. He was the first stud of note.....whom was able to pass his genotype onto his get.
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In 1872, the Yorkshire Terrier was introduced into the United States, and was recognized by the AKC in 1878. But it wasn’t until the 1930’s that the Yorkshire Terrier took on its modern look.
It is important to note that the Yorkshire Terrier up until the 1930’s usually weighed approximately 30 pounds, not the 3 to 7 pounds it does today I mostly agreed with your post until this statement....nope not true. Even in the 1860-1870's there were classes of Yorkies divided by size......1) over 5 lbs and 2) under 5 lbs. By the 1870's this dog had already been discovered the the elite/rich ladies and was highly desired as a lap dog. The dogs which arrived in England from Scotland during the great famine.....HAD to be able to fit thru a 7 inch hoop. You see the pheasants (surf crop tenders) of the country side in Scotland (cir 1830's to 1860ish)....by law were not allowed to own a larger hunting type dog. So they developed their own small dog.....to run the fields for rats AND to sleep in bed with them at night to protect the "surfs" from rats. The Sheriff would come by quite frequently to test the size of the "little rat/vermit killers" any who could not fit threw the hoop were taken away.....and yes these dogs were largerly developed from an assortment of Terriers of the day....but when they arrived in England....they were still called "broken haired terriers". I have the FIRST stud and show records from the AKC....era 1872 - 1886.....and even then here in the United States they were still be shown in two classes.....over 5 lbs and under 5 lbs. I can send some of these records if anyone wishes to see. they are very interesting. <g>
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So if I understand this right - eventually we will breed these dogs down to less than a few oz. 
It used to be that 7 lbs was OK and now we are into the 3 or 4 lbs. At what point do we stop? With the mass breeders doing it for the $$$ and people paying top $$$$ these dogs will get smaller and smaller and smaller.
Someone the other day made a comment about the standard just saying it couldn't be over 7#'s but what people are forgetting is there is a height requirement.
According to the offical written standard (you can view it on the AKC and YTCA website) there is no height requirement in the "standard" and never has been since the very first written standards of the Yorkie...going back into the early years when they were first accepted as Yorkshire Terriers. It's important to note there are no disqualifications (DQ's) in the Yorkie written standard....as there are in other breeds...this is one of the reasons breeding Yorkies is so difficult.....The only reference at all to height is
Body
Well proportioned and very compact. The back is rather short, the back line level, with height at shoulder the same as at the rump.
and
Weight
Must not exceed seven pounds.
Maybe the below is a written standard from another registery....APRI...CKC ?? dont know.....but it's certainly not included in the YTCA approved written standard.
Size and Appearance
The Yorkshire Terrier is a long-coated small-size toy dog. The weight of these dogs should not exceed 7 lb.,
the height should be 6-7 inches. The long body coat is glossy, fine, silky and straight. Hair on the muzzle is very long.
Very good points tho....
D