556 



SCIENCE 



[N. S. Vol. XXIX. No. 744 



Type III. Tilted fault blocks. — Many great 

 series of step faults are hard to explain by the 

 idea of collapse of an uplift. In some eases the 

 inclination of the strata require that the uplift 

 have a height far exceeding any known elevation 

 on the earth, yet there is no physiographic evi- 

 dence that such elevations existed. It is more 

 probable that the tilting and faulting of the 

 blocks were concomitant. The hypothesis of con- 

 tinental creep can not explain some of the phe- 

 nomena. 



A suggestion is furnished by a faulted pebble 

 in schist, collected by J. S. Diller. (See U. S. 

 Geological Survey Bulletin 353, fig. 4, p. 22, and 

 Americwn Journal of Science, Vol. XXIV., fig. 5, 

 p. 12, July, 1907.) According to Becker the 

 schistose structure is in the final direction of 

 maximum slide, and the oblique faults are in 

 another final direction of maximiun slide subor- 

 dinate to the first. Both were initiated by the 

 same rotational shear or scission, which tilted the 

 oblique faults forward, and which produced prac- 

 tically no change in the direction of the schistose 

 structure. 



A cross-section of the mountains of Colorado 

 bears a strong resemblance to this pebble. The 

 schistose structure or direction of greater motion 

 corresponds with the thrust faults and hypothet- 

 ical buried sheer zones of the moimtains. The 

 cross-breaks correspond with the step faults ef 

 South Park and the Leadville District. The 

 initial formation of the breaks on the principles 

 of cleavage laid down by Becker, and the subse- 

 quent forward rotation of the fault blocks by the 

 same shearing thrust, would explain all the phe- 

 nomena. In this way both the thrust faults and 

 the normal step faults could be produced at the 

 same time by the same forces. 

 Quartis as a Geologic Thermometer: Fbed. Eugene 



Weight and Espeb S. Labsen. 



On any temperature scale certain temperatures, 

 as the boiling and freezing points of water, are 

 arbitrarily chosen as fixed and standard points of 

 reference. For the geologic temperature scale, sim- 

 ilar points must be selected and for this purpose 

 melting points of minerals and mineral aggregates 

 (eutectics), and especially inversion temperatures 

 of enantiotropic forms of the same compoxmd, may 

 serve. Quartz is well adapted to furnish one and 

 possibly two such points, since it has two inver- 

 sion temperatures, the one at about 560°, where 

 a-quartz inverts to ;8-quartz, and the second at 

 about 800°, where /3-quartz changes under certain 

 conditions to tridymite. Miigge' has recently 



shown that at 560° quartz changes to a second 

 form called /3-quartz, which is also hexagonal and 

 trapezohedral in its symmetry but in all proba- 

 bility hemihedral instead of tetartohedral. This 

 change in symmetry class involves certain changes, 

 as crystal habit, character of twinning and inter- 

 growth of right- and left-handed quartz, and 

 fracturing of crystals, which in turn can be used 

 directly to distingvdsh quartz formed above 560° 

 from that which has never reached that inversion 

 temperature. These criteria were applied to 

 quartz from 43 different localities, 17 of vein 

 quartz, 13 of pegmatite and 13 of granite and 

 granite porphyry quartz. Nearly 500 plates of 

 quartz in all after the basal pinacoid were cut, 

 polished, etched and tested from these view-points 

 — the net result of the investigation being that 

 the vein quartzes were formed below 560° ; also 

 the quartz of certain pegmatites, while all granite, 

 granite porphyry and graphic pegmatite quartzes 

 were probably formed above 560°. By thus fixing 

 temperature limits of formation of quartz, it is 

 possible in many instances to determine limits 

 for other minerals associated with quartz. 



The Stream Rohhery on which the Belle Fourche 

 Reclamation Project is Based: N. H. Daeton, 

 U. S. Grcological Survey. 



The Belle Fourche project provides for the 

 irrigation of 85,000 acres of the Great Plains 

 lying north of the Black Hills in western South 

 Dakota. The water is to be taken from Belle 

 Fourche River just below the town of Belle 

 Fourche, carried down the north bank a short 

 distance, and then by a deep cut through a nar- 

 row divide to a large reservoir sustained by a 

 long dirt dam. From the reservoir it will be car- 

 ried by ditches into a large, low-lying basin, 

 where it will be utilized. As in most reclamation 

 projects, the topography presents certain favor- 

 able peculiarities. In this case it is that the 

 river near Belle Fourche is considerably higher 

 than the wide basin of tributaries on the opposite 

 side of a narrow divide. This basin was excavated 

 in soft shale by creeks of moderate size. Orig- 

 inally the present river was a very small branch 

 creek, but having greater declivity, it finally cut 

 back through a narrow divide and tapped the 

 headwaters of the Little Missouri River. The 

 river has not yet greatly deepened its valley near 

 Belle Fourche, so that now the water can easily 

 be carried by a short canal system into the large, 

 low lying basin to the north. 



^"Neues Jahrbuch," Fest band, 181-196, 1907. 



