She did a lot of worrying over immodest bathing suits, and never went near the beach except as a member of a purity committee, to see how awfully young girls behaved in those public places.
She let Jack have the car only because she believed that he was going to take a party of young Christian Endeavorers up Mount Wilson to view the city after dark.

She could readily apprehend that such a sight might be inspiring, and that it would act as a spur upon the worthy ambitions of the young men, urging them to great achievements.

Mrs.
Singleton Corey had plenty of enthusiasm for the betterment of young lives, but she had a humanly selfish regard for the immaculateness of her new automobile, and she feared that the roads on the mountain might be very dusty and rough, and that overhanging branches might snag the top.

Jack had to promise that he would be very careful of overhanging branches.
Poor lady, she never dreamed that her son was out at Venice gamboling on the beach with bold hussies in striped bathing trunks and no skirts; fox-trotting with a brown-eyed imp from the telephone office, and drinking various bottled refreshments--carousing shamelessly, as she would have said of a neighbor's son--or that, at one-thirty in the morning, he was chewing a strong-flavored gum to kill the odor of alcohol.
She was not sitting up waiting for him and wondering why he did not come.

Jack had been careful to impress upon her that the party might want to view the stars until very late, and that he, of course, could not hurry them down from the mountain top.
You will see then why Jack was burdened with a sense of deep responsibility for the car, and why he drove almost as circumspectly as if he were sober, and why he would not join in the hilarity of the party.
"Hist! Here comes a flivver!" warned the young man on the front seat, waving his revolver backward to impress silence on the others.