de Generatione et Corruptione 330b31-331a23

• The simple bodies, since there are four, fall into two pairs which belong to the two regions, each to each; for Fire and Air are forms of the body moving towards the limit, while Earth and Water are forms of the body which moves towards the center. Fire and Earth, moreover, are the extremes and purest; Water and Air, on the contrary, are intermediates and more combined. And, further, the members of either pair are contrary to those of the other, Water being contrary to Fire and Earth to Air; for they are constituted from contrary qualities. Nevertheless, since they are four, each of them is characterized simply by a single quality; Earth by dry rather than by cold, Water by cold rather than by moist, Air by moist rather than by hot, and Fire by hot rather than by dry.
  
• It has been established before that the coming-to-be of the simple bodies is reciprocal, At the same time, it is manifest, on the evidence of perception, that they do come-to-be; for otherwise there would not have been alteration since alteration is change in respect to the qualities of the objects of touch. Consequently, we must explain what is the manner of their reciprocal transformation, and whether every one of them can come-to-be out of every one--or whether some can do so, but not others.
  
• Now it is evident that all of them are by nature such as to change into one another; for coming-to-be is a change into contraries and out of contraries, and the elements all involve a contrariety in their mutual relations, because their distinctive qualities are contrary. For in some of them both qualities are contrary--e.g. in Fire and Water, the first of these being dry and hot, and the second moist and cold; while in others one of the qualities is contrary--e.g. in Air and Water, the first being moist and hot, and the second moist and cold. It is evident, therefore , if we consider them in general, that every one is by nature such as to come-to-be out of every one; and when we come to consider them severally, it is not difficult to see the manner in which their transformation is effected. For, though all will result from all, both the speed and the facility of their conversion will differ in degree. 

This passage is about the four elements, Earth, Air, Fire, and Water. It is relatively straightforward and does not seem to me to require extensive notes and analysis on my part. There are some big questions about how what he talks about could actually work, but it seems to me what he is saying is relatively clear.