The author does not appear to describe this granary in his text, but the image shows people storing rice and threshing it. Rice was one of the staples of Madagascar in the mid-nineteenth century. William Ellis (1794–1872) was an English missionary and author, who went to Madagascar on three occasions in the 1850s.
William Ellis, Three visits to Madagascar during the years 1853-1854-1856 (New York, 1859), p. 314. (Copy in Special Collections Department, University of Virginia Library)
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"Rice Granary", Slavery Images: A Visual Record of the African Slave Trade and Slave Life in the Early African Diaspora, accessed September 18, 2020, http://www.slaveryimages.org/s/slaveryimages/item/2585