On the last two installments of this four-part series, Thrice almost completely tosses the striking, guitar-heavy sound for which it's known. For "The Alchemy Index Vols. III & IV: Air & Earth," the material is appropriately lofty to represent the former element but surprisingly sparse for the latter. "A Song for Milly Michaelson" and "Daedalus" convey the polarized, meandering spirit of "Air," which sometimes drifts gently and sometimes blows in hard gusts. "Earth" is even more delicate than "Air," with cuts like "Digging My Own Grave" and "The Lion and the Wolf" set to spartan piano lines. "Moving Mountains" and "Come All You Weary" push farthest experimentally, verging on bluegrass and country twang. —Christa L. Titus