this first one is an Australorp chick...you have either a Barred Rock, or Maran as stated from previous person...
Here are my two new Barred Rocks, they are NOT pure...if they were, the lighter one would be the rooster, the darker, the hen. As it stands I don't know exactly what they are, I am guessing the darker is the roo based on leg size and large comb. These pictured are 5 weeks. How old is yours?
This picture is my first attempt, a few years ago at pullets...I bought nine...ALL nine ended up being roos...so here are both Barred Rock and Australorp roos....and one final pic for you, right before we got rid of all of them...
Yours may be a Maran....I have never owned one, I know it can be tricky to determine the difference, maybe someone else can chime in.