1. Heinrich, P.V., 2007a, The Giant Concretions of Rock City Kansas. BackBender's Gazette. vol. 38, no. 8, pp. 6-12
and 2. Heinrich, P.V., 2007b, Megaspherulites. BackBender's Gazette. vol. 38, no. 7, pp. 8-12.

The former, Heinrich (2007a), can be downloaded as apart of BackBender's Gazette. vol. 38, no. 8 and the latter,
Heinrich (2007b), can be downloaded as apart of Bender's Gazette. vol. 38, no. 7.

The Le site d'Irna has two web pages about natural stone balls (stone spheres), which are cannonball concretions. They are:

1. All that nature can never do, part IV : stone spheres [Le site d'Irna].
2. Stone balls : in France too! [Le site d'Irna].