Thousands of migrants were trying to find a way across Turkey’s western border with Greece Monday, with only dozens managing to pass through either border fences or fording the river there, after Turkey opened its side of the frontier to migrants and refugees to leave the country for Europe. Greek police made use of tear gas against the crowds trying to push through. Other migrants were trying to reach Greek islands from the Turkish coast, with one dinghy capsizing, leaving one child dead, Greek authorities said.