Complex sentence: The pre- "r" vowels in words like "mirror" and "nearer" are merged in some General American accents, usually to; the quality of the vowel before is even more variable in the word "miracle".
Split into simpler sentences: The pre- "r" vowels in words like "mirror" and "nearer" are merged in some General American accents, usually to. The quality of the historic "mirror" vowel is even more drastically variable in the word "miracle".