function getElementsByClass (className) {
  var all = document.all ? document.all :
    document.getElementsByTagName('*');
  var elements = new Array();
  for (var e = 0; e < all.length; e++)
    if (all[e].className == className)
      elements[elements.length] = all[e];
  return elements;
}

function showLayer(layerId){
	var maskItems = getElementsByClass("showContent");
	for(i=0;i<maskItems.length;i++){
		var maskItem = document.getElementById(maskItems[i].id);
		maskItem.style.display = 'none';
	}
	var show = document.getElementById(layerId);
	show.style.display = 'block';
}