Fahrenheit to Celsius converter won't output the correct values? [C++]

Im new to C++ so I am trying to do small projects to help me learn. When I try to run my program, the fahrenheit values print properly, but the celsius value prints as a series of numbers and letters. Here is my code

#include <iostream>
using namespace std;

double conversion( double& fahrenheit, double celsius)
{
    celsius = (fahrenheit - 32.0) * (5.0/9.0);
    return celsius;
}

int main()
{
    double fahrenheit;

    for ( fahrenheit = -40.0; fahrenheit <= 220.0;)
    {
        cout << fahrenheit << "F = " << conversion << "C" << endl;
        fahrenheit = fahrenheit + 10.0;
    }
    system( "pause" );
    return 0;
}