W. B. Emery — Igneous Geology of Carrizo Mountain. 363 



underlying Jurassic (?) sediments is, however, excellently ex- 

 posed and may be considered typical. Study of a series of thin 

 sections from specimens collected at various distances from 

 this contact revealed a slight increase in the amount of iron 

 oxide in the cement of the sandstone directly at the contact. 

 If, however, one did not know that such an amount of iron 

 oxide was unusual in this sandstone he would not recognize the 

 sandstone as metamorphosed in the least, and, indeed, the 

 amount of metamorphism has been so small that at a distance 

 of three feet from the contact the bed is absolutely normal. 



On the mountain summit metamorphism was somewhat more 

 potent inasmuch as the original sandstone has been changed to 

 a resistant quartzite, and this to an unknown though probably 

 not great distance from the contact. 



Sandstone columns at Holmes Dike contact. — Where mel- 

 anocratic types of rock have penetrated the sedimentary beds, 

 metamorphism, though not of great importance, is more pro- 

 nounced than that produced by the porphyry. In the major- 

 ity of cases a baking of the enclosing sediments has accompanied 

 the intrusion of basic types of rock in this region, but Holmes 

 Dike (fig. 3) has produced certain quartzite columns similar in 

 appearance to those commonly observed in igneous outflows. 

 Study of a thin section of one of these quartzite columns 

 demonstrated the absence of any admixture of igneous material, 

 that the columns consist only of Jurassic sediments altered to 

 quartzite. The columns are of various diameters to one and 

 one-half inches and in length range to three feet, the maxi- 

 mum distance of metamorphic action. Similar, though less 

 perfect, columns were observed by the writer 35 miles west of 

 Carrizo Mountain along the contact of a dike associated with 

 Boundary Butte, Utah ; more perfect columns have been noted 

 by Professor Gregory at the northern end of Lukachuka 

 Mountain, Arizona.* 



Cause of the absence of intense contact metamorphism. 



It is evident from the above discussion that there has been 

 no great amount of contact metamorphism in the Carrizo area, 

 since such effects are commonly not noticeable at a distance of 

 more than three feet from the contact. This phenomenon has 

 previously been recognized by Cross as characteristic of lacco- 

 lithic intrusions in the southwest and has been attributed by 

 him to the lack of the so-called mineralizing agents, fluorine, 

 chlorine, and superheated steam in the magma. f The absence 

 of intense metamorphism in Carrizo Mountain is believed to 

 be due to this cause. 



*Prof. Paper U. S. Geol. Survey, in preparation. 



■(•Cross, Whitman, Spencer, A. C, Purington, C. W., La Plate folio (No. 

 60), Geol. Atlas, U. S., U. S. Geol. Survey, 1899, p. 11. 



