It has long appeared to me that much remains to be said on this subject. When Mr. Croll's papers on the Physical Cause of the Motion of Glaciers appeared in the Philosophical Magazine for March, 1869, and September, 1870, I felt that, though his theory was certainly ingenious, and his statement of it clear and masterly, these papers were far from affording a satisfactory solution of the problem of which they treated. I waited, fully expecting that Mr. Croll would supplement and modify his theory, but he has not done so to my knowledge. Geologists in general seem to be as satisfied with it as he is himself, and on all hands they appeal to it as supporting them, whenever they treat of glacial phenomena, even when, if true, it shows their theories to be vain. A rather conspicuous instance of this occurs in the Geol. Mag. for October of last year, in an interesting paper by Mr. Goodchild on Coums. In that paper Mr. Goodchild commends “a proper appreciation of Mr. Croll's theory”; but I am persuaded that a proper understanding of it would have caused him either to renounce that theory or his own on the origin of Coums. I have thought it time, therefore, to place on these pages my criticisms and difficulties for the consideration of geologists, even if, as is unfortunately too likely, I have little new to advance.