multiple layers: gridLayer and tileLayer

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Burlington Geographic - Physical Maps</title>

var baseLayer = new L.mapbox.tileLayer('landplanner.hl6099hm').addTo(map);

var ui = document.getElementById('layerControls');

// Define the layers for the map
addLayer(L.mapbox.tileLayer('vanhoesenj.VTBedrock'), 'Bedrock Geology', 1);
addLayer(L.mapbox.tileLayer('vanhoesenj.VtSurfGeo'), 'Surficial Geology', 2);
addLayer(L.mapbox.tileLayer('landplanner.hli55fb7'), 'Soil Types', 3);
addLayer(L.tileLayer('https://s3.amazonaws.com/geosprocket/btvgeographic/{z}/{x}/{y}.png'), 'Elevation Contours', 4);

function addLayer(layer, name, zIndex) {
    layer
        .setZIndex(zIndex);

// Create a simple layer switcher that toggles layers on
    // and off.
    var link = document.createElement('a');

// Define button element properties
    link.href = '#';
    link.className = 'btn btn-primary';
    link.type = 'button';
    link.innerHTML = name;

// Update map and element properties by selected layer
    link.onclick = function(e) {
        e.preventDefault();
        e.stopPropagation();

if (map.hasLayer(layer)) {
            map.removeLayer(layer);
            this.className = 'btn btn-primary';
        } else {
            map.addLayer(layer);
            this.className = 'active btn btn-primary';
        }
    };

ui.appendChild(link);
}

// Add a reference overlay
var topPane = map._createPane('leaflet-top-pane', map.getPanes().mapPane);
var topLayer = new L.mapbox.tileLayer('landplanner.hl60jemk', {
  maxZoom: 18
}).addTo(map);
topPane.appendChild(topLayer.getContainer());
topLayer.setZIndex(7);
</script>

</body>
</html>

Modified http://api.tiles.mapbox.com/mapbox.js/v1.6.2/mapbox.js to a secure url

Modified http://d3js.org/topojson.v1.min.js to a secure url

https://api.tiles.mapbox.com/mapbox.js/v1.6.2/mapbox.js

https://d3js.org/topojson.v1.min.js