The slabs and nodules pictured below were called snowball agate or snowballs. According to "The Agates of Northern Mexico" by Brad Cross they were found west of Hy. 45 and north of Villa Ahumada about 60 miles south of El Paso Tx. The nodules were small and round with a white to tan soft outside that had the appearance of a "ball of caliche" when freshly mined. The nodules were sold under a couple of other names as well (David Polson). The post by Trista Morton may be one of these nodules and if so, is one of the best I have seen.
Snowballs - largest about 2 1/2 ins