282 Reviews — The Geology of Pigeon Point, Minnesota. 



igneous origin, and the doming of the rocks is attributed to the force 

 of crystallization bf minerals from supersaturated solutions. The 

 petroleum, however, is probably of later date. Since for various 

 reasons ordinary mining is impossible, the sulphur is extracted by 

 the Frasch process, which consists essentially in forcing superheated 

 water through a pipe to the sulphur horizon and lifting the sulphur 

 by means of compressed air, when it is pumped into large bins to 

 cool and then loaded straight into trucks. Under the present 

 abnormal conditions it is possible that much of this exceptionally 

 pure sulphur will have to be used for the manufacture of sulphuric 

 acid, an uneconomic proceeding which would be quite unjustifiable 

 in normal times. T? H "R 



III. — The Geology of Pigeon Point, Minnesota. By R. A. Dalt. 



American Journal of Science, vol. xliii, p. 423, 1917. 

 rnHIS paper gives the results of a further investigation of the 

 .L well-known intrusion of Pigeon Point, originally described by 

 Bayley (Bulletin 109, U.S. Geol. Survey). The main problem is as 

 to the nature of the "red rock", a granitoid type with much 

 micropegmatite. The igneous mass is concluded to be a sill, not 

 a dyke as supposed by Bayley ; its upper and lower surfaces are 

 found to be concordant with the bedding of the Animikie rocks, into 

 which it is intruded. The most striking feature is the regular 

 variation in the character of the rock from below upwards ; the base 

 is olivine-gabbro, followed by gabbro, intermediate rock, and finally 

 the highly acid red rock. This variation in composition is due to 

 gravitative differentiation, actihg on a gabbroid magma which had 

 been modified by assimilation of sedimentary material derived from 

 the roof of the sill by stoping. The differentiation is probably 

 largely due to gas-action : the presence of much gas is shown by 

 the abundance of drusy cavities, and peculiar "ribbon" injections, 

 a few centimetres wide, also indicate great pressure in the magma. 

 In all the rock-types are found xenoliths of Animikie quartzite 

 coated with a shell of red rock. This suggests a diffusion of silica 

 from the xenolith into the basic magma. The red rock appears to 

 have remained liquid longer than the more basic types, owing to 

 concentration in it of gas, which lowered its freezing-point. 

 Consequently dykes and veins of red rock are found cutting the 

 gabbro as well as the sediments. The red rock is not merely fused 

 sediment as supposed by Bayley. jS'either diff'erentiation nor fusion 

 alone is sufficient to explain all the facts, and the whole phenomenon 

 is an example of syntexis, that is assimilation followed by differentia- 

 tion of the mixed magma thus formed. "R TT "R 



lY. — On a possible Causal Mechanism for Heave-fault Slipping 

 in the California Coast-range Region. By H. 0. Wood. 

 Bull. Seismol. Soc. America, vol. v, p. 214, 1915. 



rpHIS paper has special reference to the origin of the fault-slip 

 X. that caused the disastrous San Prancisco earthquake of 1906. 



The theory is based on the principle of isostatic readjustment 



