The symbolic heap is widely used in verification of pointer-manipulating programs because it supports forward symbolic execution. While it can describe disjoint data structures easily, it is not suitable to describe overlapping data structures. Based on the observations that overlapping structures are often immutable in practical programs, we propose read-only assertions and extended separation logic to solve the problem. Our logic simplifies the specifications and verifications of overlapping structures. We formalize our logic, its soundness proof, and program verification examples in Coq.