Dr. Croll's “Climate and Time” is perhaps the most important geological work that has appeared in recent years, certainly the most valuable and original that refers to Glacial Geology. It teems with novel and instructive ideas. The part which is best known and has chiefly contributed to its celebrity is the theory, propounded and maintained in the first four chapters, of the causes of the Glacial Epoch. Though the theory met with much opposition at first, it seems now to be gradually gaining belief. Not only are Dr. Croll's views accepted by geologists, but his arguments are quoted in text-books. Now, when Science is used as a means of education, it is of high importance that the reasoning placed before the student should be sound. I propose in this article to examine Dr. Croll's description of the manner in which, according to his views, Glacial Periods have been brought about, to discuss the soundness of his reasoning, and to investigate the actions of the causes which he suggests. Some of the remarks here made are probably originally derived from an able article by Mr. S. V. Wood, jun., in the Geol. Mag. for 1876. The suggestion at the end of the paper is due to Mr. W. M. Hicks, M.A., Fellow of St. John's College.