Complex sentence: Cassin was one of the leading mountaineers of the inter-war period; in all, Cassin made a total of 2,500 ascents, of which over 100 were first ascents.
Split into simpler sentences: Cassin was one of the leading mountaineers of the inter-war period. In all, Cassin made a total of 2,500 ascents, of which over 100 were first ascents.