32 PRINCIPLES OF STRATIGRAPHY 



layers, as in Mt. Starr-King, California, or Mt. Monadnock, New 

 Hampshire ; or by breaking off of fragments of all sizes, which 

 cover the mountain top to the frequent exclusion of solid ledges, as 

 on Pike's Peak, Colorado ; Mt. Adams, New Hampshire ; Sugar 

 Loaf Mountain, Brazil ; Ben Nevis, Scotland ; and most mountain 

 summits. Such fragments, supplemented by those broken off by 

 frost action, accumulate at the foot of every cliff and on every 

 steep slope, forming the talus, the surface angle of which varies 

 with the coarseness of the material. (See Chapter XIIT.) 



The best illustration of such disruption of rocks is seen in 

 desert regions, where changes of temperature are very great. Liv- 

 ingston has described the disruption, with ringing sound, of the basalt 

 masses in South Africa. Other examples are cited from Brazil and 

 the Atacama desert of Chile, where the temperature in winter ranges 

 from — 12° C. at 7 a.m. to +37° C. at 11 a.m., and the summer 

 temperature from -|-5° C. to -|-55° C. From tropical west Africa 

 Pechuel-Loesche* has recorded temperature ranges of 60° to 84° C. 

 (\'Valther-i04 1-'^" et scq.; also 103:55(5-557 and literature there 

 cited.) Walther has found this shattering of rocks through insola- 

 tion in limestone, flint, sandstone, porphyry, granite, gneiss, quartz, 

 and other rocks. The crack penetrates gradually into the depths of 

 the rock, so that half-broken pebbles are not infrequently found. 

 Such cracks are visible in rocks of all sizes, from fragments as large 

 as a nut to blocks of the dimensions of a house. In limestone and 

 granites, Walther often found peripheral cracks which permitted the 

 .separation of concentric shells, varying in thickness from that of a 

 sheet of paper in some limestones to shells 10 cm. thick in some 

 granites. In Brazil the range of temperature is considerably more 

 than 100° P., while in the northern United States -it is 150° F. 

 ( — 30° to -)-i25°) in half a year, with a diurnal variation often 

 amounting to half that. (Shaler.) 



Changes of temperature in like manner affect the individual 

 mineral constituents of the rock, as well as their association in the 

 rock. Since each mineral has its normal coefficient of expansion and 

 contraction under a given influence of heat or cold, it follows that 

 irregular stresses and strains are set up within a rock mass of rela- 

 tively coarse-grained minerals such as a granite, and that this must 

 lead to a slipping back and forth of the minerals upon each other 

 with ultimate disruption or disintegration of the mass as a whole. 

 Walther regards the diff'erence of color of these minerals as of 

 especial significance (103:55(5), and Branner (lo) has also called 

 attention to the fact that coarse texture likewise favors this type of 

 disintegration. This is well shown in coarse granites of sub-arid 



