Select All of an Object Type

When the user has an object selected, there should be a select all "x" object option. Then the user could adjust or delete all of a given item. For example, when I generate a forest and I think there are too many rocks but like the forest otherwise, I could select a rock, click this "select all rock" button and hit the trashcan, getting ride of all that type of rock from the map. Another example of it being useful is if I want to scale the size of all of a certain item, I could select all, turn off collisions, and change the size of everything at once instead of one at a time.