Restricting 'add-hook to a specific file extension

[Tim Johnson <tim@johnsons-web.com>]

Is there a way to restrict 'add-hook to a specific file extension?
    
Below is code from my .emacs:

(require 'newlisp-mode++) ;; uses 'add-hook to extend 'scheme-mode
(add-to-list 'auto-mode-alist '("\\.lsp\\'" . scheme-mode))
(autoload 'scheme-mode "scheme" "Turn on scheme mode" t)
(add-hook 'scheme-mode-hook 'turn-on-font-lock)

I only want 'newlisp-mode++ to be used when the file extension is ".lsp". If
the file extension is ".scm", I would want 'scheme-mode, but I would *not* want
the code from 'newlisp-mode++ to take effect. 

One solution is derived-mode, but I've had no luck with it. For the time being
(until I've had another 6 months or so of elisp under my belt), is there a way
to restrict the add-hook action to only a file with a .lsp extension? 

Thanks
tim

-- 
Tim Johnson <tim@johnsons-web.com>
      http://www.alaska-internet-solutions.com