The lots or tracts of land whereon Fort Mechanic is erected and such other lots and parcels of land as may be considered necessary to that establishment have been ceded to the United States for all purposes necessary to the maintenance of a military post. But nothing in such cession shall be construed to prevent any process, civil or criminal, issuing from any of the courts of this State or any other competent authority from being served or executed within the limits of such lots or tracts of land nor to impair the rights and privileges vested in the city council of Charleston, under their charter of incorporation. But said city council may relinquish its right or jurisdiction in and over the land aforesaid.