Tafsir of Al-Hijr 15:74

(So We made its highest part)—that is, the city, as is apparent; though it is also permissible that it refers to the villages, even if they were not previously mentioned—(its lowest). The "highest part" refers to the surface of the earth and what was upon it, and this is the first object of the verb "to make" (ja‘alnā), and "its lowest" is the second object. The discussion regarding this has already preceded.

(And We rained upon them)—in the midst of that—(stones)—existing (of Sijjīl).

(Of petrified clay); according to the well-known opinion, it is an Arabized form of sang-i gil. Abu ‘Ubayd and a group maintained that it is Arabic, and that it is also said as "Sijjīn" with an n. They cited as evidence the saying of Tamīm ibn Muqbil: A striking that the champions urged upon one another, [like] Sijjīn. This is as you see it. Al-Asma‘i was asked about its meaning in the verse, and he said: "I will not interpret it, for I heard it when I was a youth as sakhīnā (with a dotted khā’)," meaning hot. It is also narrated as Sijjīn with a jīm.

It is also said: It is derived from al-sijill (the scroll/book), meaning: from clay upon which their names were written, or upon which Allah wrote their punishment. The discussion regarding this has also already passed.