To break so vast a Heart — Heart-Wood

To break so vast a Heart ¹
Required a Blow as vast－
No Zephyr felled this Cedar straight－
'Twas undeserved Blast－

(F.1308/J.1312)

[1] Heart:: heart-wood.
[3] Zephyr:: soft wind, breeze. Cedar:: an evergreen tree.
[4] undeserved Blast:: alluding a strike too vast that shouldn't happen to Dickinson, who has an evergreen heart. Breeze-like strike can't hurt her.

Heart-wood, which we know is drier, harder, more solid, and much more durable as timber, than sap-wood. It is generally of a different color, and it exhibits in different species the hue peculiar to each, such as reddish in Red-Cedar, brown in Black-Walnut, black in Ebony, &c. ─ First Lessons in Botany and Vegetable Physiology (1864)