The Northwestern library system consists of four libraries on the Evanston campus including the present main library, University Library and the original library building, Deering Library; three libraries on the Chicago campus; and the library affiliated with Garrett-Evangelical Theological Seminary. The University Library contains over 4.9 million volumes, 4.6 million microforms, and almost 99,000 periodicals making it (by volume) the 30th-largest university library in North America and the 10th-largest library among private universities. Notable collections in the library system include the Melville J. Herskovits Library of African Studies, the largest Africana collection in the world, an extensive collection of early edition printed music and manuscripts as well as late-modern works, and an art collection noted for its 19th and 20th-century Western art and architecture periodicals. The library system participates with 15 other universities in digitizing its collections as a part of the Google Book Search project. The Mary and Leigh Block Museum of Art is a major art museum in Chicago, containing more than 4,000 works in its permanent collection as well as dedicating a third of its space to temporary and traveling exhibitions.

Answer this question, if possible (if impossible, reply "unanswerable"): How does the University Library rank among North American university libraries?
the 30th-largest