Complex sentence: The 13 British colonies in North America used the avoirdupois system, but continued to use the British system as it was, without the evolution that was occurring in Britain in the use of the stone unit.
Split into simpler sentences: The 13 British colonies in North America used the avoirdupois system. But they continued to use the British system as it was, without the evolution that was occurring in Britain in the use of the stone unit.