"Extraordinary Holmes! Why would anyone put these cards in our mailbox?"
"Let's see, Watson. Ah, I think this is what Lestrade was supposed to be sending, he's trying to find Professor Moriarty's hidden treasure."
"Indeed! Well, he could have packaged them better, they have got quite shuffled, I'm afraid."
"A pity certainly Watson, but let's see what can be made of them. Mrs. Hudson! I shall need my best pipe tobacco!"
"Well, each card has the name of a chemical element, if I'm not mistaken. And I see two different inks were used so I can divide these into two sets. Now this set is interesting, I think this must involve our Colonial friends."
"Amazing, Holmes, how could you possibly deduce that?"
Holmes laid the cards on the table:
Aluminum
Fluorine
Molybdenum
Nitrogen
Rhenium
Technetium
"You see that, Watson? It's spelled 'A-L-U-M-I-N-U-M', quite definitely American.
And I see more. Mrs. Hudson! Do we have an atlas of California?"
"And what do you make of the other set, Holmes?"
"Well that seems more difficult..."
Again, he laid the cards on the table:
Erbium
Gallium
Hydrogen
Magnesium
Phosphorus
Radon
Rubidium
Yttrium
"Here's the atlas, Mr. Holmes."
"Thank you, Mrs. Hudson, now let me see..."
Holmes thumbed through the atlas, then reached across for a chemistry text he kept at hand.
"Ah yes, of course! Mrs. Hudson, I won't be needing that pipe tobacco!"
"Why ever not, Holmes?"
"Because, my dear Watson, it's elementary!"
Note: The roads close to Washington are a mess, drive carefully.