In today's cultures, adoption is talked of openly, but in genealogy, it can create a little havoc. Many people kept mum about adopting a baby or putting their child up for adoption. Seldom did the paperwork qualify that it was an adoption. The language of adoption was very vague. Many charitable organizations were set up to take care of orphaned children or children whose parents could no longer take care of them. There records give a lot of family information.
Many times children were taken in by whatever family was available after the death of their parents from disease or accident. If the children were young enough they might never know they were adopted, or who their birth parents were or came from.
If you are interested there is a wonderful compilation of wagon train diaries and journals at http://www.over-land.com/diaries.html and this site also has links to courses on understanding the 18 &19 century language and handwriting, which could come in very handy when reading from old journals and family bibles.
Researching a modern day adoption can be much more difficult. There are many parties involved you need to consider and it is possible the birth mother has no wish of being located plus the adoptive parents may feel slighted by the desire to find the birth parents. So step gently when you begin this type of research!
GM & CB