From the Wikipedia page [1]
Alger is a former French department in Algeria. The département of Alger existed between 1848 and 1962.
Considered as a French province, Algeria was departmentalised on 9 December 1848, thereby operating according to the same administrative structure as metropolitan France. Three civil zones (départements) replaced the three beyliks into which the Ottoman former rulers had divided the territory. The principal town of the central département, also called Alger, became the prefecture of the eponymous département. The two other Algerian departments were Oran in the west and Constantine in the east.
The département of Alger covered an area of 54,861 km², and comprised six sub-prefectures: these were Aumale, Blida, Médéa, Miliana, Orléansville and Tizi-Ouzou.
It was not until the 1950s that the Sahara was annexed into departmentalised Algeria, which explains why the département of Alger was limited to what is the north-central part of Algeria today. Until 10 January 1957, when the Sahara regions received their own administrative structure, these territories were administered by the département of Alger.
On 26 January 1956 population increases triggered the creation of three new stand-alone departments. These were the département of Médéa, along with the coastal départements of Orléansville and Tizi-Ouzou formed respectively from the southern, western and eastern portions of the département of Alger.
The département of Alger remained in existence after the independence of Algeria and became Algiers Province in 1968.