If you are trying to Stack your Array from the Bottom..Use Push to back....
Your Tiles are made up of I presume for terrain stuff like Grass, dirt , rock, etc and you want it done in layers?
There would need to be some limited Randomization to your Generation seed but no so much that its too chaotic...this takes time to figure out...Seeding is a fairly exact science..you need to know the basics...its not something you will pick up in 5 min....You will get the concept but not the actual coding..
You probably should start a proper Grounding in the idea... before you get serious..its a complex field but easily doable once you understand what has to happen in order to do it..
Read up as much as you can then do some tests...before you make your game..every hours you spend studying will make your game better in the end..
Try these for starters and run with it...
http://www.gameprogrammer.com/fractal.html
http://en.wikipedia.org/wiki/Category:Computer_graphics_algorithms
Also this might help you...
GENERATING RANDOM TILE
The basic idea is to Generate a seed , then load it into the Array either from front to back or vice versa....persona preference for me is start at the base. the ROOT data and then move up the tree creating nodes of detail as you require..
this way..the over all effect is a coherent world populated by detailed areas with Gold, lava, rock, gems etc...that are created and populated at the right place and layer ..this is all determined by your seed or Ruleset that you create and must know how this works before you can really even start to think about it.
Load your tile data or create a generation algorithm that generates the tiles you need and then "push to Back all of the Data in the right sequence...so obviously Grass would go at top..Dirt next, then rock etc...but yeah..STUDY first ..then test, then refine...