How do i detect if a part has a part under it?

I am relatively new to roblox scripting, and I am making a terrain generation Script. I have looked up how to do it but no results. This would be really helpful because I need dirt to be under grass. Here is my script

--Variables--

mapxsize = 256
mapysize = 32
mapzsize = 256
seed = math.random(0, 1000000)
noisescale = 30 
amplitude = 20
blocksize = 2 

--The Script--

for x = 0,mapxsize do
    for z = 0,mapzsize do
        for y = 0,mapysize do
            xnoise = math.noise(y/noisescale,z/noisescale,seed)*amplitude
            ynoise = math.noise(x/noisescale,z/noisescale,seed)*amplitude
            znoise = math.noise(x/noisescale,y/noisescale,seed)*amplitude

            density = xnoise + ynoise + znoise + y

            if density < 18 and density > 12 then
                part = Instance.new("Part",workspace.TerrainFolder)
                part.Anchored = true
                part.Size = Vector3.new(blocksize,blocksize,blocksize)
                part.CFrame = CFrame.new(x*blocksize,y*blocksize,z*blocksize)

            end
        end
    end
    wait()
end