Narrative passages, on the...
To be effective, descriptive passages usually incorporate the five senses in imagery: taste (gustatory), smell (olfactory), hearing (auditory), touch (cutaneous or tactile), and sight (visual). Also, other senses include kinesthetic (movement), vestibular (balance and eye coordination), and organic/subjective (sensations such as fear, joy, anger, etc).
Narrative passages, on the other hand, tell a story of some sort. The story may be told from the vantage point of personal experience, or an author may present the private thoughts of a character to his/her readers. Either way, these literary aids help move the plot forward or allows us to learn something significant about the character in question.
On to Germinal:
Descriptive passages: Bolded words in the two passages below are mine.
The distant hammer struck (auditory) regular blows in the pit, and the wind passed by with its moan, like a cry of hunger and weariness (auditory) coming out of the depths of the night.
Etienne, who forgot himself before the stove, warming his poor, bleeding hands (visual) looked round, and he could see each part of the pit: the shed tarred with siftings,, the pit frame, the vast chamber of the winding machine, the square turret of the exhaustion pump. This pit, piled up in the bottom of a hollow, with its squat brick buildings, raising its chimney like a threatening horn, seemed to him to have the evil air of a gluttonous beast crouching there to devour the earth. (auditory, visual, kinesthetic, subjective/organic).
M. Hennebeau, who was at this hour returning home mounted on his mare, listened to these vague sounds. He had met couples, long rows of strollers, on this beautiful winter night. More lovers, who were going to take their pleasure, mouth to mouth, behind the walls. Was it not what he always met, girls tumbled over the bottom of every ditch, beggars who crammed themselves with the only joy that cost nothing? And these fools complained of life, when they could take their supreme fill of the happiness of love? Willingly would he have starved as they did if he could begin life again with a woman who would give herself to him on a heap of stones, with all her strength and all her heart. His misfortune was without consolation, and he envied these wretches.
In the story, M. Hennebeau is a part owner of the Le Voreux mining operation. The mine is the setting for the strike; throughout the novel, the mine is characterized as a place of misery, degradation, and oppression. However, the passage above gives us an insight into M. Hennebeau's character, his personal trials, and his desires. The thoughts above show a man, vulnerable in his private moments, far from being only the inexorable manager who refuses to allow any concessions to his workers. Narratives like the above tend to humanize an unlikable character.
Hope this helps! There are more such passages in the novel. With the above as a guide, you should be able to recognize many more on your own.