REVIEWS 163 



and was not due to any intrusive or crystalline force inherent in the deposit 

 itself. Some, however, instead of being simple sheets between well-defined 

 layers of rock, split and reunite, forming between and about cracked and riven 

 layers of rock, leaving it less clear that all the mechanical action preceded the 

 deposition of the ores. In other cases "dice mineral" and blende impregnate 

 soft beds of rock in sheetlike belts, in which instances it is quite evident that 

 the metaliferous substances displaced the yielding rock in the process of their 

 growth. (P. 468.) 



Upon close inspection the soft clayey rock will be found to be thickly inset 

 with scattered crystals of black blende, giving to the fractured rock a beautiful 

 speckled appearance. Looking still closer, it will be seen that the laminae of 

 the rock curve around the particles of blende, showing that they were displaced 

 by the growth of the blende crystals. (See Fig. 39.) We have in this and 

 the next instance just as clear cases of the forming mineral making room for 

 itself by its own concretionary force, as in some preceding cases it is evident 

 that the receptacle was first formed and the ore subsequently implanted. (P. 474.) 



Two other cases are cited, pp. 474, 475, and p. 464. 



The first edition of Branner and Newsom's syllabus of economic 

 geology, published in 1895, referred (p. 28; p. 36 of the second edition, 

 1900) to the enlargement of veins by accretion, under the following heads: 

 "Illustration of needle-ice and crystallization in the soil. The size, form, 

 and structural relations of some geodes shown to be due to enlargement. 

 Evidences of mechanical force of the process. Possible relations to vein 

 enlargement; to brecciation." 



That same year, 1895, the fourth edition of Dana's Manual of Geol- 

 ogy appeared, in vv^hich Professor Dana briefly mentions (p. 138) "dis- 

 placement by intrusion of crystalline material," and cites Worthen upon 

 the splitting and enlargement of crinoids. The paper of Worthen has 

 not been located by the writer. In 1899 Professor Shaler published 

 a paper upon the formation of dikes and veins, ^ in which he uses geodes 

 as illustrations of vein-forming, and remarks (p. 262) that "the pressure 

 of the growing vein .... is likely to be even more effective in the group 

 of tabulate deposits in forcing the walls asunder." This is the only 

 instance, with which the writer is acquainted, in which the relation of 

 geode formation to veins has been formally discussed. 



In the twentieth annual report of the United States Geological Survey, 



Part II, published in 1900, Professor I. C Russell says in regard to the 



filling of certain brecciated veins (p. 207): "I venture the suggestion 



that these minerals (quartz and calcite), in crystallizing, have exerted a 



I Bulletin of the Geological Society of America, Vol. X, pp. 253-62. 



