While cleaning the basement, scrubbing the walls, concrete patch started crumbling off, revealing a wooden board. further study revealed there was a hollow space behind it. Seeing as how this was the base of the foundation, I removed the board for repair. after removal, I discovered that the rest of the base was concrete, yet only 3-5 inches thick, to a height of a little over a foot. there, I found the base of the original stone foundation. seeing as the rest of the basement wall is poured concrete, with the exception of most of this wall, which is stone and mortar. this meets poured concrete near this corner. also, this wall does not mark the edge of the house, there is a crawlspace with dirt floor extending past the wall. My questions follow- 1-why is most of this 1 wall stone, not poured concrete? 2-is the 3-5 inches of concrete that runs the length of the base sufficient for safety? 3-is this something I can repair myself? and if so, some guidance would be appreciated I have attached several pics of the problem.