The siege eventually failed when a large relief force approached and forced Henry to withdraw. In 1790, Montmartre was located just outside the limits of Paris. That year, under the revolutionary government of the National Constituent Assembly, it became the "commune" of Montmartre, with its town hall located on "place du Tertre", site of the former abbey. The main businesses of the "commune" were wine making, stone quarries and gypsum mines. (See Mines of Paris). The mining of gypsum had begun in the Gallo-Roman period, first in open air mines and then underground, and continued until 1860. The gypsum was cut into blocks, baked, then ground and put into sacks. Sold as "'montmartarite", it was used for plaster, because of its resistance to fire and water. Between the 7th and 9th centuries, most of the sarcophagi found in ancient sites were made of molded gypsum. In modern times, the mining was done with explosives, which riddled the ground under the butte with tunnels, making the ground very unstable and difficult to build upon. The construction of the Basilica of Sacré-Cœur required making a special foundation that descended 40 metres under the ground to hold the structure in place. A fossil tooth found in one of these mines was identified by Georges Cuvier as an extinct equine, which he dubbed "Palaeotherium", the "ancient animal". His sketch of the entire animal in 1825 was matched by a skeleton discovered later. Russian soldiers occupied Montmartre during the battle of Paris in 1814. They used the altitude of the hill for artillery bombardment of the city. Montmartre remained outside of the city limits of Paris until January 1, 1860, when it was annexed to the city along with other communities ("faubourgs") surrounding Paris, and became part of the 18th arrondissement of Paris. In 1871, Montmartre was the site of beginning of the revolutionary uprising of the Paris Commune. During the Franco-Prussian War, the French army had stored a large number of cannon in a park at the top of the hill, near where the Basilica is today. On 18 March 1871, the soldiers from the French Army tried to remove the cannon from the hilltop. They were blocked by members of the politically-radicalised Paris National Guard, who captured and then killed two French army generals, and installed a revolutionary government that lasted two months.