Exclusive jurisdiction over the land so purchased, and to be purchased, is hereby ceded to the United States by the Commonwealth of Pennsylvania, and said lands shall be exempt from the payment of all taxes, state and local: Provided, That the Commonwealth of Pennsylvania shall retain concurrent jurisdiction with the United States over the lands so acquired by the United States, as far as necessary, for the purpose of serving all civil and criminal processes which may be issued under the authority of the Commonwealth in causes arising without the area so acquired.