Minecraft wiki entry for Cobblestone_farming, associated text: The block marked ! is where stone is generated and moves as shown. A trench beneath the stone path will contain unfortunate lava spills.
The glass block needs some gravity block atop it: Sand is shown, gravel or even concrete powder will also work.
This build can be mirrored but not rotated: It will only work with the output going east or west, apparently due to a south-east rule.