Tweeted by Timothy Noah:

"Although just 7-8 percent of France’s population is Muslim, as much as 70 percent of prison population is Muslim." https://t.co/PCm4kfkKm5 — Timothy Noah (@TimothyNoah1) November 16, 2015

Here's the NYRB quote in context—they seem to think that this is France's fault:

Today, France has one of the largest Muslim minorities in Europe. French Muslims are also predominantly a social underclass, a legacy of France’s colonial past and indifference to its aftermath. For example, although just 7 to 8 percent of France’s population is Muslim, as much as 70 percent of the prison population is Muslim, a situation that has left a very large number of young French Muslims vulnerable to absorbing radical ideas in prison and out. Within this social landscape, ISIS finds success. France has contributed more foreign fighters to ISIS than any other Western country.

These guys aren't an underclass because of French racism, but because they're like that. Allow them to emigrate to the US, and they'll be l ike that in your town.