The existence of rocks possessing, when the laminæ are not too thick, the property of flexibility has long been known. Upon flexible sandstone (“Itacolumite”) a great deal has been written at different times, and of late years important work has been done which renders it necessary to greatly modify the opinions formerly held with regard to this rock. Notwithstanding the interest which, is attached to the subject, it is one very much neglected by our textbooks, the British, either ignoring it altogether, or treating it with the utmost brevity, the German, while sometimes referring to it at considerable length, do not do more than enunciate the old views. To Prof. Judd—who has kindly aided me with advice, and by affording facilities for preparing this paper—I owe the suggestion that it would therefore be useful to give a résumé of the present state of our knowledge upon the flexibility of rocks in general.