Air consists of combination of nitrogen and oxygen held in place by gravity

1/ Air consists of combination of nitrogen and oxygen hold in place by gravity
=> I personally think that there are two mistakes in that sentence: combination and hold, which should be changed into “a combination” and “held”

2/ we arrived after electricity had been cut and now John was lighting the oil lamp
=> do you find the sentence awkward? I feel it very strange with the phrase “now John was lighting the lamp” => both the past continuous tense and the use of “now” here…

1/ You’re absolutely right.

2/ For formal English it is a little awkward, but in colloquial English, or more vivid literary English, it’s perfectly fine. All it’s doing is shifting your point of view so that you are in the seen and imagining John at the very time he was lighting the oil lamp.