Of the deserts on the America Continent, which one covers the greatest area?
SELECT T5.Name FROM country AS T1 INNER JOIN encompasses AS T2 ON T1.Code = T2.Country INNER JOIN continent AS T3 ON T3.Name = T2.Continent INNER JOIN geo_desert AS T4 ON T4.Country = T1.Code INNER JOIN desert AS T5 ON T5.Name = T4.Desert WHERE T3.Name = 'America' ORDER BY T5.Area DESC LIMIT 1