After you pass through the north part of Route 112, which you reached via Fiery Path, you arrive in the ash-covered Route 113.
Meet Your Neighbor
You will see your neighbor near the entrance to the route. Your neighbor heals up your pokémon, mentions Professor Cozmo in Fallarbor, then leaves.
Explore the Route
In this route, you can catch Spinda, Sandshrew, and Skarmory. Skarmory is very hard to catch, so you should get Ultra Balls from Mauville, or wait until you reach Fallarbor and get some Timer Balls to have a better chance of catching a Skarmory.
Be sure to catch at least two Spinda while you are here.
Go west and you will see volcanic ash start to fall from the sky.
To the west, you find Youngster Neal, who has Trapinch and Electrike.
Go up the nearby stairs and use your Dowsing Machine to find a hidden Ether in a pile of ash.
If you go northeast, you can hop down the ledge and battle Fairy Girl Franny, who has a Kirlia.
Near the Fairy Girl, there is a pile of ash. If you walk through the pile, you reveal a Super Repel.
If you go back up the stairs near the Youngster, there is a pile of ash to the left of the stairs. If you walk through it, Ninja Boy Lao pops out and challenges you to a battle. He has three Koffing.
To the west you find Parasol Lady Madeline, who has Numel.
There is a Secret Base entrance near the Parasol Lady. Remember that you need to teach Secret Power to a pokémon in order to open up the Secret Base.
Go up the stairs near the Parasol Lady. To the east, there is TM32 Double Team.
To the west of that TM, you find Youngster Dillon, who has Aron.
To the west of the Youngster, you find the Glass Workshop. Go inside and talk to the owner to receive the Soot Sack. To collect ash, just walk through the piles of ash in the route. A random amount of ash (between 3 and 100 grams) will be added to the Soot Sack. The ash piles reappear whenever you leave the route and come back. There are trainers hiding in a couple of the ash piles here, so be prepared. The piles to avoid are the one east of the Parasol Lady, and the northern pile of ash that is surrounded by grass to the south of the Glass Workshop.
After you have collected enough ash, go back to the Glass Workshop owner and he will give you items in exchange for the ash. There are five glass flutes that you can get, and two secret base decorations.
Walk through the ash pile in front of the Glass Workshop to find a Max Ether.
Go west from the Glass Workshop and go to the northwest ash pile. Walk through it to find a Hyper Potion.
The ash pile to the south of this one has a trainer hiding in it: Ninja Boy Lung, who has Nincada and Ninjask.
Now go west to reach Fallarbor Town.