Mormon is speaking of the small plates. It is somewhat curious that he prays for their preservation, since he has included them in his own set of plates, for which he also prays for preservation. However, this may be formulaic rather than specific. The intent is that the words of the plates come to future generations, and perhaps Mormon foresaw that the inclusion in his set of plates might be seen in such a way that they might would have been skipped (which appears to have been the pragmatic case, where the Book of Lehi was followed in the translation sequence by the Book of Mosiah). He is therefore praying that the words might come forth to future generations, rather than be concerned for the physical preservation of the plates themselves.