January 7, 1916] 



SCIENCE 



25 



limestone at the time it was deposited. He 

 therefore regards the magnesia as indigenous. 

 It has appeared to the writer that the agents 

 which produced the mottling might be closely 

 bound up with dolomite formation on an ex- 

 tensive scale, and he has accordingly given 

 the phenomenon careful attention in connec- 

 tion with his studies on the origin of dolomite. 

 In the occurrences of mottled limestones ob- 

 served by him the dolomite patches follow 

 fucoid markings similar to those described by 

 Wallace in some instances, but in others they 

 are very irregular and show no guiding influ- 

 ence. For the origin of both types it seems 

 necessary to adopt an alternative hypothesis; 

 namely, that the magnesia was subsequently 

 introduced into the limestone from without, 

 and that the mottling has resulted from the 

 selective replacement of fucoid markings in 

 the one case, and from the spreading out of the 

 alteration from certain favorable centers in the 

 other. Consistent with this view are the fol- 

 lowing facts: 



1. The existence of unaltered fucoid mark- 

 ings containing less than two per cent, of 

 magnesium carbonate in association with 

 dolomitic ones. 



2. The association of both types of mottling 

 with dolomite seams and other evidences of 

 imperfect dolomitization. 



3. The graduation of mottled beds into beds 

 which are uniformly dolomitic, both laterally 

 and vertically. 



4. The existence of every gradation between 

 limestone showing incipient mottling and true 

 dolomite. 



Thus it appears to the writer that all ex- 

 amples of mottling examined by him repre- 

 sent an incipient stage in the process of dolo- 

 mitization, and it is believed that many dolo- 

 mites have passed through such a stage in the 

 progress of their formation. Here, then, we 

 have a clue to the origin of aU those masses 

 of dolomite with which such mottling is as- 

 sociated. 



With regard to the time of the alteration 

 which produced the mottling, there is con- 

 vincing evidence that it took place in the ma- 

 jority of cases prior to or contemporaneously 



with the recrystallization of the limestone. 

 Several features lend support to this conclu- 

 sion; namely, the development of perfect 

 rhombs of dolomite showing no growth inter- 

 ference effects in the limestone about the bor- 

 ders of the dolomite patches; the occasional 

 presence of zonal growths of dolomite and cal- 

 cite; the tendency of the dolomite areas to 

 spread out uniformly in all directions as the 

 dolomitization proceeded rather than to de- 

 velop veinlets ; and the association of the mot- 

 tling with imperfect dolomitization effects 

 along original lines of weakness such as bed- 

 ding planes rather than along secondary struc- 

 tures such as joints or fractures. It seems 

 probable, therefore, that the mottling was pro- 

 duced while the limestones were still beneath 

 the sea, and that the sea water contributed 

 the magnesia. Francis M. Van Tuyl 



University of Illinois 



serpent instinct in man 



To THE Editor op Science: In the very 

 entertaining and instructive work by Col. Wm. 

 C. Gorgas, " Sanitation in Panama," the 

 author in the concluding pages of the book 

 gives expression to certain philosophizing 

 ideas relating to the earliest period of the 

 existence of the human race, and makes the 

 point that before the discovery by primitive 

 man of fire and clothing his habitat must have 

 been confined to that part of the earth that lies 

 " between the tropics of Cancer and Capri- 

 corn," or within narrow limits outside of that 

 region. 



There has been much speculation concern- 

 ing the focus from which the world's popula- 

 tion became diffused over the earth's surface, 

 and, at least as far as regards the white peo- 

 ples of Europe and Asia, the consensus of 

 scientific opinion has fixed upon some locality 

 in central Asia as the probable focus of origin, 

 though the exact or approximate locality seems 

 not to have been defined. 



In the writer's reflections along this line 

 there has presented itself to his contemplation 

 one very pronounced and mysterious mental 

 attribute still pertinaciously clinging to the 

 white race at least, which seems to carry evi- 



