712 



SCIENCE. 



[N.S. Vol. XXV. No. 644 



Valley. It is most probable, however, that the 

 mountain is underlain at depth by igneous 

 intrusions. 



At the close of the Cretaceous period, at 

 the time of the great Laranude or Rocky 

 Mountain uplift, the sea bottom was uplifted 

 into land, as a part of the Mexican Plateau 

 province, and the rocks were wrinkled and 

 compressed into the marvelous overthrown 

 folds which they now present, having north- 

 east and north-south trends. 



This period of folding was that of the 

 whole of the eastern Eocky Mountain region 

 in north-south directions, and the union of 

 the North and South American continents. 



During the later epoch, probably the 

 Miocene, the region and its limestone rocks 

 underwent another erogenic movement, and 

 was intruded from below by a vast igneous 

 magma from which came dikes and ejecta of 

 volcanic rocks which are now found through- 

 out the region. Accompanying this epoch 

 the whole structural or tectonic trend of the 

 southern continent changed from a north- 

 south to north 40° west course, and the north 

 40° west faults and folds of this later erogenic 

 revolution were developed across the older 

 north-south folds and faults, resulting in the 

 development of the quaquaversal structure of 

 northern Mexico, and I have seen it at La 

 Mitra, Villadama and Oandela, in the State 

 of Coahuila, the Big and Little San Vincente, 

 mountains crossed by the Eio Grande at 

 Presidio San Vincente; at Guaynopita, Jesus 

 Maria and Santa Rosalia, Mexico; and at 

 Monument Mountain near El Paso, and in the 

 Santragos Chinati and other mountains of 

 Trans-Pews, Texas. The great mineral lodes 

 of Mexico were mostly intruded, as hot vapors, 

 waters and gases up these northwesterly fault 

 zones. 



Since or during this second epoch, the 

 plateau as a whole has been uplifted, increas- 

 ing the erosion, and the surface has worn 

 down at least 2,000 feet. Some of the mineral 

 contents have been washed away; others have 

 concentrated down the fractures enriching the 

 present ore bodies. 



As the writer has previously shown, nearly 

 all of the great ore localities of Mexico are 



associated with faults in this north 40° west 

 direction. These faults are aU of late geo- 

 logical origin, and probably the movements of 

 the earth which made them are still going on, 

 as testified by hot springs, in the vicinity. 



By plotting the trends of the faults and 

 folds of the Cordilleran region upon a large 

 map, such as cannot be produced here, it cer- 

 tainly appears as if the northern part of the 

 Mexican plateau province from the Colorado 

 plateau southward through the northern tier 

 of the States of Mexico has been the site of 

 the crux of two distinct periods of moun- 

 tain making movements, as illustrated in the 

 Sierra Almoloya. One of these, and the older, 

 has northerly trends representing the Lara- 

 mide movements of the typical Rocky Moun- 

 tain type, and the other having the north- 

 western trends of the Coast Range type, prob- 

 ably representing a post-Miocene revolution. 

 The hypothesis of the crossing of these two 

 belts of deformation explains many of the 

 hitherto unexplained phenomena of our con- 

 tinent along the International Boundary and 

 in Northern Mexico. It is certainly worthy 

 of further test by field observation. Further 

 study may show that the post-Miocene belt of 

 northwest-southeast movement crossing the 

 Laramide or Eocky Mountain north-south 

 movement may extend from California across 

 Mexico and connect with the Antillean move- 

 ments of the West Indies. 



Robert T. Hill 



New York City 



TBE BIOLOGICAL LABORATORY OF THE 



U. 8. BUREAU OF FISHERIES AT 



WOODS HOLE, MASS} 



Investigators and Assistants.— Dnring the 

 past laboratory season, thirty-two investigators 

 were engaged in the study of various phases 

 of marine biology, this number being some- 

 what larger than has been recorded for any 

 summer within the last four years. Of these 

 investigators, fourteen received a salary from 

 the bureau, while eighteen are to be classed 

 as volunteers. Thirteen junior assistants 

 were also employed for various duties in the 

 laboratory and in the field, two of whom, from 

 the nature of part of their work, have like- 



' Report of the work for the summer of 1906. 



