(Portland, OR)  --  Police believe a string of property fires that took place in Northwest Portland are cases of arson.  Authorities say the fires happened between two and four o'clock yesterday morning, and the items set afire included a Jeep Wrangler, shopping carts, a Porta Potty, and recycling bins.  The blazes kept fire crews busy for several hours.  Anyone who has information on the fires is being urged to call 503-823-3700.