return or log return

Hi there, I downloaded 2 sets of financial data from yahoo finance earlier.

I am calculating the daily returns by using the opening price and closing price (CP-OP)/CP........Now when I log this I get negative values even though I have positive returns on some days. Additionally it does not allow me to compute correlation and coefficient.

Can anybody tell me if I am doing something wrong or should I not take the log of the return?

Even when I log CP/OP it still gives bad values.