CC Ādi 5.60

kṛṣṇa-śaktye prakṛti haya gauṇa kāraṇa
agni-śaktye lauha yaiche karaye jāraṇa
Word for word: 
kṛṣṇa-śaktye — by the energy of Kṛṣṇa; prakṛti — the material nature; haya — becomes; gauṇa — indirect; kāraṇa — cause; agni-śaktye — by the energy of fire; lauha — iron; yaiche — just as; karaye — becomes; jāraṇa — powerful or red-hot.
Translation: 
Thus prakṛti, by the energy of Lord Kṛṣṇa, becomes the secondary cause, just as iron becomes red-hot by the energy of fire.