Is there any law that specifies how long an employer has to put your money into your 401k after deducting it?

My employer makes bi-weekly deductions from my pay check based on the amount I specify. He then holds that money (and presumably my other coworkers' contributions) for several months before sending it to be deposited into my 401K account. This sounds illegal to me. What if the stock market surged and all my money wasn't in my account to take advantage of it?