Do you mean stromatolite? If so, a stromatolite is a structure formed from layers of sediment cemented together by the biofilm of a certain microorganism, such as cyanobacteria. Ancient stromatolites are fossils of one of the very first organisms on Earth. Modern stromatolites are also found in a few select locations around the world
Source: My head - I have a Biology degree and stromatolites were covered in a variety of my undergrad courses. More info can be found at Wikipedia - http://en.wikipedia.org/wiki/Stromatolite