Keep in mind I'm speaking for the urban wastelands of my own country only, in particular the nation's capitol (ironically, also the captitol of Europe) where 2/3 of the population is of immigrant (mostly Northern-African) descent (incidentally, some parts of the capitol have 96% population of immigrant descent). Basically, in many cases it goes like this: little or no education -> no job prospects -> descent into life of crime and/or leeching off the social services.
Most third generation immigrants also usually are arranged to marry a person of the countryside of their home-country, which is then brought to the country. This person usually only speaks the language of the home country only and is unfamiliar with Western culture, continuing the cycle.
(There's of course also the cluster**** of French-speaking fleeing Brussels en masse and moving to Dutch-speaking communities bordering the nation's capitol, taking them over, but that's a different matter entirely. Let's just say if the Dutch-speaking weren't mellowed by centuries of being conquered and down-trodden by pretty much every major country, there would've been quite the historyof civil wars here).