In Old Portsmouth in Broad Street on the water side of the road by the Round Tower.

HMS Hecla
During the Russian War (1854) a landing party
From HMS HECLA was attacked by a large body
Of Cossacks and many would have fallen had it
Not been for the courage of two sailors who
Taking cover behind this stone kept the enemy at
Bay until the safety of the whole party was assured
Captain HALL had this boulder carried to his ship and
Transported to Portsmouth.

Further Information
Although we have no further information concerning this memorial, HMS Hecla was party to another notable event in that same year. The following is an extract from 'The VC at Sea' by John Winton.
The earliest act of bravery to win a Victoria Cross was carried out by Charles Davis Lucas, a twenty-year-old Mate (what would now called a Midshipman) serving in the 6-gun steam paddle sloop HM Hecla with the Baltic Fleet, under Admiral Sir Charles Napier, in the summer of 1854. Napier was a highly popular admiral but he and his fleet had come under public and parliamentary criticism for lack of success, against the enemy. The Russians were content to stay in harbour, sheltering inside their coastal fortresses, one of which was at Bomarsund, in the Aland Islands, guarding the entrance to the Gulf of Bothnia.
Although it was not normally good tactics for ships to attack heavily defended shore forts, the Navy had to find targets where they could, and on 21st June Captain W.H. Hall commanding the Hecla led his ship and the two 16-gun paddle-steamers Odin and Valorous through the narrow channel to Bomarsund. The ships were fired on by riflemen and artillery from the shore and the main fort batteries also opened fire, somewhat prematurely, betraying their position. All three ships anchored at about nine o'clock that evening (it was midsummer and light in those latitudes until nearly midnight) and began an intermittent but spirited bombardment which lasted until one o'clock the next morning, but without (as it later transpired) doing much lasting damage. Hall was later commended by the King of Sweden but criticised by the Admiralty for using so much ammunition.
At the height of the bombardment, a live shell from an enemy battery landed on Hecla's upper-deck, with its fuse sdll hissing. All hands were ordered to fling themselves flat on the deck, but Lucas, with what Hall called in a letter to Napier next day 'great coolness and presence of mind', ran forward, picked up the shell and tossed it overboard. It exploded with a tremendous roar before it hit the water. Some minor damage was done to the ship's side and two men were slightly hurt but, thanks to Lucas, nobody was killed or seriously wounded. He was immediately promoted to Acting Lieutenant for his bravery, and the Admiralty later confirmed the promotion on Napier's strongest recommendation. Lucas's Cross was gazetted in the first list of 24th February 1857, and he was present at the first Investiture to receive his Cross from Queen Victoria in Hyde Park on 26th June that year. (See also: http://www2.prestel.co.uk/stewart/kent.htm)
Can we have our rock back?
In 1965, the Royal Navy launched a new ocean survey vessel which was named HMS Hecla, the eighth to be so named. When it was realised that the ship was to be commanded by Captain GPD Hall, namesake of the Captain mentioned above, the Navy suggested to the City Council that it would be fitting if the boulder could be removed for permanent display on the quarter deck of the new ship. Amazingly the Council offered no objection to this proposal, but luckily the MoD realised that carrying a rock of such a weight on board ship could cause serious problems and never persued their enquiry. [See Council Minutes 1965 Min. 617]