Complex sentence: Layard provided an opportunity for Rassam to travel to England and study at Oxford (Magdalen College), where he stayed for 18 months before accompanying Layard on his second expedition to Iraq (1849--1851).
Split into simpler sentences: Layard provided an opportunity for Rassam to travel to England and study at Oxford (Magdalen College). He studied there for 18 months before accompanying Layard on his second expedition to Iraq (1849--1851).