Mr. C. S. Wilkinson has contributed some valuable matter to Cave literature. His account of the Jenolan Caves, written for the Government, contains several beautiful passages bearing upon the physical and chemical agencies at work in the formation of limestone caverns. And here it may be interesting to refer to one or two other authorities also on the same subject. Dr. Wright, an American scientist, says, "There can be no doubt that the solvent action of water holding carbonic acid in solution" is the primary agency concerned in the formation of limestone caves. "Limestone," he says, "is not soluble in water until it combines with an additional proportion of carbonic acid, by which it is transformed into the bicarbonate of lime. In this way the process of excavation is conducted until communication is established with running water, by which the mechanical agency of that fluid is made to assist the chemical. Little niches and recesses, which seem to have been chiselled out and polished by artificial means, were formed in this manner; for when these points are strictly examined, a crevice will be observed at the top or at the back of them, through which water issued at the time of their formation, but which has been partially closed by crystals of carbonate of lime or gypsum." Dr. Wright, referring to the different conditions and different periods of cave formation, says:--"The sulphate of lime, which is known under the name of gypsum, plaster of Paris, selenite, alabaster, etc., exerts a much greater influence in disintegrating rock than the sulphate of soda. The avenues in which gypsum occurs are perfectly dry, differing in this respect from those that contain stalactites. When rosettes of alabaster are formed in the same avenue with stalactites, the water which formed the latter has for ages ceased to flow, or they are situated far apart, as the former cannot form in a damp atmosphere."