solecism

[…] to suppose that the general concern of this Country can be directed by thirteen heads, or one head without competent powers, is a solecism, the bad effects of which every Man who has had the practical knowledge to judge from, that I have, is fully convinced of; tho' none perhaps has felt them in so forcible, and distressing a degree.

The "simple" metaphor, such as the instance given, becomes the "continued" metaphor when the analogy or similitude is worked out in a series of phrases and expressions based on the primary metaphor; it is in such "continued metaphors" that the solecism of "mixed" metaphors is likely to occur.

"Best young woman!" repeated Mr. Lovel; "'pon honour, Jack, you have made a most unfortunate speech; however, if Lady Louisa can pardon you,-and her Ladyship is all goodness,-I am sure nobody else can; for you have committed an outrageous solecism in good manners."