Thanks for this Jeffrey. I've just spent the afternoon searching for details about promotion in the Marines but have drawn a blank, so I think we'll have to let the matter rest for now.

However I did find the following snippet which gives an interesting perspective on the Marines. It is from a review on the Times Higher Education website of a book entitled "The Royal Marines: From Soldiers to a Special Force", author Major-General Julian Thompson:

"Marines of the 18th century were by no means trained as, nor intended to be, an elite force. Marines were paid less than seamen. Many marines left the corps to become able seamen and earn higher pay and higher social standing aboard ship. In the navy, neither securing a commission nor promotion was by purchase. For years, the marines attracted officers who could not afford to purchase a commission in the army. According to Thompson: "The marines became a poor man's regiment." At sea, the highest rank a marine officer could hope for was captain, possibly major. Ashore, his chances were much improved within his division, where a colonelcy might materialise, in late Victorian times even the rank of brigadier-general. The esteem in which officers and men were held was low. "