Field Mushroom? Positive ID?

4" across. Dark Brown spore, brown gills, white cap.
Never ever eat a mushroom you pick without being 100% sure what it is. At least eighteen varieties in the UK will cause you significant damage/death. A mycologist will almost always want a spore print to confirm ID, which is why I usually take one - you lay the mushroom down gill-side down on a sheet of white/black paper and leave for a few hours. The print in this case was produced within three hours.
A useful site ot help with identification is www.rogersmushrooms.com/gallery/chooser.asp