Thanks for your question about this pretty wildflower. It is Mentha aquatica, or water mint, an introduced herb that grows in wet spots, often in meadows. As you've seen, it makes a lovely haze of violet when it is in bloom. If you get close enough, you will detect the smell of mint. Although it spreads by rhizomes as all mints do, it is not considered invasive in this area, probably because it is confined to seasonally damp areas.
I enjoy the natural color harmony between the flowers of water mint, and the lavender flowers of teasel, which blooms at the same time (and is considered a noxious weed, but is striking nonetheless).

The plant in my photograph has flowers all along the stem.  When I researched Water Mint, all show the flowers only at the Top of the stem, and it is described as such.  Is there another variety of Water Mint like my photograph, or could this be another plant species?  It is true that I found it growing in the wet part of the field, and as you described, it makes a lovely haze of violet.  Finally, what is your opinion about introducing the plant to my flower garden?  I know that rhizomes can be difficult to control in some situations. 

It may be Mentha arvensis, field mint, which does tend to have more whorls of flowers, or even Mentha pulegium, pennyroyal. To be sure, I think it would be best to key a sample out - they are pretty similar. If you want to take a sample in to the Extension office, 4077 SW Research Way, Corvallis, they would have a reference to do that.
As to introducing any of them into your garden, I don't know of any reason not to as long as you are very aware of its running tendencies!