420 B. R. Semmes — Tertiary Intrusives of 



sandstone and conglomerates which is considered a possi- 

 ble representative of the Morrison formation. These 

 sediments lie in a structural basin and show frequent 

 intrusions of igneous rock in the form of dikes and sills. 

 The upper member of the Carboniferous series, the San 

 Andreas limestone, is found out-cropping at varying dis- 

 tances from the coal field, but at no point was it found 

 directly associated with the Cretaceous strata. 



Tertiary (?) Conglomerates. — In the vicinity of Melena 

 Station about 15 miles northeast of Roswell there occur 

 massive beds of poorly consolidated gravel, locally known 

 as "concrete", which lie unconformably above the Red- 

 beds forming prominent mesas. This material though 

 unfossiliferous is regarded as probably of Tertiary age. 



Caliche. — The redissolved and poorly cemented lime- 

 stone material that rests unconformably upon the older 

 rocks, which has been termed caliche, is to be found cap- 

 ping the table lands throughout the Pecos Valley. Be- 

 neath the Caprock of the plains, which is of similar mate- 

 rial, the caliche is underlain by unconsolidated sands and 

 gravels of apparently Recent age. In the Pecos Valley 

 the caliche is the only formation other than the river allu- 

 vium that is not cut by the igneous intrusions. 



Intrusives. 



The igneous intrusives of the Pecos Valley in some 

 cases form pronounced topographic features and are 

 locally well known to the inhabitants of the area, who have 

 in many cases prospected their contacts for the precious 

 metals, but they have never been fully described in any 

 publication with which the writer is familiar. 6 These 

 intrusives appear to be confined to the area north of Ros- 

 well, though the source or center of igneous activity from 

 which they are derived is still uncertain. The two dikes 

 are almost exactly parallel and extend eastward until cov- 

 ered by the drifting sands in the area just west of the 

 Caprock, so that it is not all impossible that they might be 

 derived from some center of igneous activity now con- 

 cealed beneath the Plains. On the other hand other intru- 

 sives are to be found west of the Pecos and there also 



6 The dikes east of the Peeos Elver have been mentioned by C. A. Fisher : 

 Geology and Underground Waters of the Koswell Artesian Area, New 

 Mexico, U. S. Geol. Survey, Water Supply Paper 158, 1906. p. 8. 



