NORTH AMERICAN GEOLOGY. 225 



valuable than those reprfseuted by the hypothetical speculations com- 

 posing the greater part of the past literature on this subject. 



104. Becker's memoir on the Cretaceous metamorphic rocks of Cali- 

 fornia will prove especially interesting among those which have ap- 

 peared during the past year, for his results have been obtained by ex- 

 ceptionally detailed work with every possible auxiliary in a country 

 which is stated to be particularly easy of study. It has been found 

 that sandstones and arkoses of Neocomian age liave been altered to 

 crystalline rocks only differing from those of the Archean by holding 

 plagioclase instead of orthoclase, andby irregularity of their alteration. 

 They include metamorphic diabases and diorites and vast quantities of 

 serpentine. These changes have been traced from slight alteration, 

 through all stages of the obliteration of evidence of plastic character, 

 to the final crystalline products. The great masses of serpentine are 

 found to be derived from the sandstones boch directly and through in- 

 termediate granular metamorphics : " Highly inclined sandstones strike 

 into serpentine areas in sucli a manner as to wholly preclude the sup- 

 position that the serpentine is an older mass, and instances are ob- 

 served where one side of an anticlinal is serpentinizcd while the other 

 is unaltered and carries excellent fossils. These relations are par- 

 ticularly clear at Knoxville and Mount Diablo." In discussing the 

 causes of these changes it is thought that they were effected in great 

 part at the time of upheaval by solutions from the underlying granites. 

 It is supposed that these were at first warm and basic and supplied the 

 material for the change to augitic and amphibolic holocrystallines. 

 Serpentinization appears to have followed at a lower temperature, and 

 finally the greater part of the silica was deposited.* 



105. The Trappean and Serpentinous rocks oc urring in a considera- 

 ble area west of Baltimore have been studied in great detail by G. H. 

 Williams, who finds them to consist of what are termed " hypersthene 

 gabbros," gabbiodiorites, peridotites and their alteration and paramor- 

 phic products. Tae first is a finegrained, purplish-black aggie ateof hy- 

 persthene, diallage, and plagioclase. The second is a dark green rock of 

 fibrous hornblende, and the third form a series characterized principally 

 by a large amount of olivine, and are refeired to the family (;f peridot 

 ites. The first two graduate into each other in the field, and microscopic 

 examination reveals the gradual transition of the second from the first 

 by change of pyroxene to fibrous-hornbende, and some other interest- 

 ing spec al features. The peridotites break through the oth r in dikes. 

 They are rarely rich in feldspar, and sometimes this is absent. By 

 gradual loss of olivine they grade into another massive rock composed 

 almost wholly of diallage and hypersthene. Thetolivine always appears 

 to alter to serpentine, and the pyroxene (no matter wliat its form) to 

 hornblende, which suffeis further alteration t > talc, with sei)aration of 

 calcite. In the main the hornblende serpentines, as those of Bare Hills, 



*Am. Jour. Sci.,.in, vol. 31, pp. 348-357. 

 H. Mis. GOO 15 



