A striking exemplification of the effects of frequent and rapid changes of temperature on rocks, and of humid and dry climates as well, is seen in the case of the great monoliths of Egypt, one of which now stands in the Central Park, New York. In the quarries of Syene in Upper Egypt, where most of these monoliths were obtained, the rough blocks that were in progress of quarrying when the work was abandoned, quite two thousand years ago, still show an almost perfectly fresh surface; and the same is true of the finished obelisks in Lower Egypt, where both the changes of temperature and the rainfall are somewhat greater. It is a matter of public note that one of “Cleopatra’s Needles” which was set up in Central Park nearly thirty years ago, but originally erected at Heliopolis on the Nile, is in great danger of destruction from the influence of a totally different climate, in which both the temperature changes and the rainfall are much more frequent and severe than in Egypt. The large crystals of feldspar and quartz which compose the (syenite) rock material have had fine fissures formed between them by often-repeated expansion and contraction; which when filled with water and subsequently changed to ice, the latter’s expansion in freezing (see below) has still farther enlarged them and caused a scaling-off, which threatens to obliterate the hieroglyphic inscriptions. Thus temperature-changes and a rain followed by freezing may in a few days produce a greater effect than a thousand years of Egyptian climate. _Cleavage of rocks._—Many kinds of rocks have definite directions of ready cleavage. The most common and obvious cases of this kind are schists, slates and shales, cleaving readily into plates or irregular flat or lens-shaped fragments. Such structure greatly favors disintegration, especially when the layers are on edge at steep angles. But there are other apparently structureless, massive rocks, particularly basalts and other eruptive rocks related to them, as well as many sandstones and claystones, that have a strong tendency to cleave into more or less definite forms when struck; such as columns or prisms, square, six-sided or diamond-shaped blocks, etc. Similar forms are naturally produced in them under the influence of changes of temperature; by the formation of minute cracks at first, then enlargement of these by the several agencies already mentioned.