Pebble-Like aggregations of calcium carbonate have been described from certain lakes in the northern United States and southern Canada under such names as squaw biscuit, marlyte, and marl balls. The most northerly of the previous records of these structures in eastern Canada, known to the writer, cites them from Emerald Lake, a small Ontario lake in latitude 47 degrees. Structures of this type are known to be products of the growth activities of certain species of the blue-green algae. Those from Canandaigua Lake, New York, have the hardness of limestone and most geologists would be likely on casual examination to class them as flat water-worn pebbles of Palaeozoic limestone. Others, such as those found in Mink Lake, near Ottawa, Canada, are much softer and less compact than limestone.