TREACLE
and
MOLASSES

The terms “treacle” and “molasses” both refer to cane syrup.   The former term is used in Britain, and the latter in North America.
        With the laudable desire to make their recipes comprehensible, many authors of cookbooks include a list of equivalents.   But nuances of meaning are often lost.
        The molasses most readily available in North America is the so-called light molasses.   It's actually dark brown; but it has been refined slightly.   The black treacle sold in Britain, on the other hand, is not refined at all.   For this reason, it is occasionally called “green” treacle—which certainly does not refer to the colour!
        What does this mean to a cook?   The flavour of unrefined cane syrup is subtly different from that of “light molasses”.   To get the authentic flavour of a British recipe, therefore, one cannot simply substitute the most commonly available molasses of North America, which is what equivalence lists would suggest.  The result will be similar in flavour; but it won't be quite right.

Instead, you should use
blackstrap molasses.

This will give you the authentic flavour.

The British actually have two common varieties of cane syrup.  Both are sometimes just called “treacle” which can be confusing, since sometimes one type is meant, and sometimes the other.   For example, the traditional dessert, “treacle tart” (a sort of syrup pie), is not usually made with black treacle.   It is made with golden syrup.