Hi Lilac...It sounds like you might have the cart there and that it is full. That could have happened and, hopefully, it did ...LOL
The following excerpt from Chris Scott's walkthrough just might answer your questions:
WARNING SPOILER:
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Pulverizing Chamber
1.Climb the ladder and walk to the far end.
2.Turn right and move toward the conveyor belt that is identical to the others you discovered elsewhere.
3.Activate the conveyor and leave the close-up, then turn right and walk to the pair of buttons mounted next to the
belt.
4.Activate the motor with the bottom button and walk to the controls for the pulverizer in the center of the cavern.
The Pulverizer Machine
(Hand/eye coordination needed here)
1.Throw the knife switch to activate the belt.
2.The right button controls the right smasher and the left button controls the left one.
3.As each boulder moves down the belt, hit the left button as it passes beneath the left smasher.
4.When it moves beneath the right smasher, press the right button.
5.Continue until the belt stops and the bell rings. This indicates the cart is full of just the right amount of the right-sized
rubble in the mining cart.
6.If you accidentally let the boulders or large rubble pass, the machine will shut off automatically. You must walk over
to the right end of the machine and pull the clutch to clear the belt. You must also walk over to the left end of the belt
and press the bottom button again to re-engage the motor and perform the process again.
7.Once you have filled the mining cart, climb down the ladder and exit the chamber.
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Love, Witchen =O)