Thirty years ago, there were dozens of variants of LISP out there, from the mainstream (MacLISP, InterLISP, Lisp Machine LISP) to the esoteric (it seems like every major CS department in the world had a local LISP dialect, much as many of them would soon have local Unix dialects).
Many of these dialects had differences between them -- from the everything's-a-message architecture of Carl Hewitt's ACTORS (think smalltalk-meets-continuation-passing-style) to a number of LISPs with logic programming features. A number of dialects even played with Algol-like syntax (McCarthy himself had planned to give LISP an Algol-like syntax. S-Expressions were a fortuitous stopping point on the way to an implementation of so-called M-Expressions which never came).
In the writings of the time, I don't see much debate over whether these systems were or were not LISP. Mostly, people understood this as a big family of languages.
In contrast, Common LISP and Scheme not only have much more in common than those LISPs did, in one important regard (lexical scoping), they are closer to each other than they are to the vast majority of historical lisp implementations. The only reason we're asking this question is because after the fading away of other historical LISP dialects, the differences between Scheme and Common LISP seem bigger than they are.
So yeah, I'd say Scheme is LISP.
(By the way, two great looks at the LISP family tree are The Evolution of Lisp[1] by Guy L. Steele and Richard Gabriel, and Olin Shivers' History of T[2], an early dialect of Scheme with an extensible version of set! similar to Common Lisp's SETF)