Well, for a start, you missed the free tea/coffee and biscuits.
Very good talk. I think the best way to describe it was that it was basically a potted summary of the archaeological work Michael Lewis has been involved in over the last 40 years in connection with the slate industry with mention of some of the aspects of the quarries he found particularly interesting.
A particular point Michael made is that some of the smaller outlying earlier quarries preserve the archaeology of their original working (late 18th/early 19th century) which, in the case of the larger quarries we're more familiar with, has been obliterated by the heavily industrialised and intensive working of the late 19th century.
Dave