Ajax to read my json string

I have a subroutinr:

 [WebMethod]
        public static string gvAjax()
        {
            List<Table1> gvlist = new List<Table1>();
            ...
            var serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
            var json1 = serializer.Serialize(gvlist).ToString();
            return json1;
        }

I get a string from the WebMethod of the form "[{"Id":1,"col1":"11","col2":"22","col3":"33"},"{"Id":2,"col1":"44","col2":"55","col3":"66"},"{"Id":3,"col1":"77","col2":"88","col3":"99"}]"

The alert gives a message of "undefined". 

How do I get Ajax to read my json string?

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script><script type="text/javascript">$(function () {$("#<%= Button2.ClientID %>").click(function (e) {
                e.preventDefault();$.ajax({
                    type: "POST",
                    url: "/Default.aspx/gvAjax",
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    data: JSON.stringify({}),
                    async: true,
                    cache: false,
                    success: function (data) {
                        alert(data.d);
                    },
                    error: function (x, e) {
                        alert("Error: " + x.responseText);
                    }
                });
            });
        });</script>