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