1 68 SIDNEY POWERS 



and against each other in ascent. The thickness of the sedimentary 

 series in the region is about 12,000 feet from the top of the pre- 

 Cambrian to the horizon of the outcrop of the dike. Hence, the 

 pre-Cambrian norite inclusions must have risen over two miles 

 and their edges would be necessarily rounded. 



The abundance of the inclusions in two dikes and a sill, while 

 other near-by dikes of the same composition and age do not show 

 any inclusions at the present exposure, is not sufficient evidence 

 to prove that all of the dikes do not contain inclusions in place. 

 The shattering of the walls of some of the dikes may have been a 

 purely accidental phenomenon, due to the way in which the fissures 

 formed through which the magma came. If the passage were 

 through a vertical fissure with straight unbroken walls, no inclu- 

 sions would be expected. 



Mancos, Colorado: Professor Kirtley F. Mather, of Fayette- 

 ville, Arkansas, has kindly contributed the following account of 

 the inclusions in dikes near Mancos, Colorado: 



In the San Juan region of southwestern Colorado, about 8 miles south- 

 west from the town of Mancos, there is a small area of igneous rock which is 

 locally known as "the Blow-out." The Mancos River at this point is flowing 

 in a steep-sided youthful valley cut through the Mesaverde formation, which 

 caps the neighboring plateaus, and far into the Mancos shale. "The Blow- 

 out" is situated high up on the eastern slope of the valley in the midst of the 

 maturely dissected shales. It consists of a conical mound about a quarter of 

 a mile in diameter and two or three hundred feet in height, composed of medio- 

 silicic extrusive volcanic material, flows, tuffs, and breccias. Cutting the 

 volcanic cone and the surrounding country rock there are several subsilicic 

 dikes which vary in width from an inch to 4 or 5 feet. The more prominent 

 of these dikes can be traced for many hundred yards outward beyond the 

 boundary of the central cone. Both the extrusive and intrusive rocks contain 

 an abundance of inclusions of several different types of rock materials. 



The extrusive volcanic rock varies considerably in its composition in differ- 

 ent parts of "the Blow-out," but typically it is an andesite consisting of 

 plagioclase feldspar, about AbjsAn^s, accompanied by biotite, magnetite, and 

 augite, imbedded in a glassy matrix. The dike rock likewise is quite variable 

 in composition, but it consists essentially of phenocrysts of biotite, augite, 

 olivine, and plagioclase, imbedded in a groundmass of glass, pyroxene, felty 

 plagioclase, magnetite, and biotite. The percentage relations of the minerals 

 in the phenocrysts is notably different in different parts of the same dike, but 

 usually the rock is rich in biotite and poor in plagioclase. The latter is little, 

 if any, 'more calcic than that in the extrusive rocks. 



