422 abstracts: geology 



In general, the cutting has been greatest along the shores having low 

 scarps made up of the clays and marls of the Talbot formation, and 

 least along those of the low-lying tidal marshes. 



A study of the submarine changes shows rather extensive scouring 

 along the eastern shore of the bay and less extensive, though equally 

 intensive, shoaling at places within the river mouth. A further and 

 more extensive study involving the entire bay and its tributary basins 

 is suggested. J. F. H. 



GEOLOGY. — Dike rocks of the Apishapa quadrangle, Colorado. Whit- 

 man Cross. U. S. Geological Survey Professional Paper 90-C. 

 Shorter contributions to general geology, 1914. C. Pp. 17-31, 

 4 plates. 1914. 



The Apishapa quadrangle is situated on the plains south of Arkansas 

 River, in Colorado, about twenty-four miles east of the mountain front. 

 The geological map of this area in the Apishapa folio, by G. W. Stose, 

 shows forty-three dikes which trend nearly west. These are a part of a 

 great system of radial dikes, with associated sills, which surround the 

 Spanish Peaks, an eruptive center situated twenty-five miles southwest 

 of the border of the quadrangle. 



The rocks of Apishapa quadrangle are all lamprophyric in character 

 and are described under the names minette, augite minette, olivine- 

 bearing augite vogesite, hornblende-augite vogesite, olivine-plagioclase 

 basalt, and sodic diabase. The series to which they belong has a much 

 greater range than this. Chemical analyses are given of four of the prin- 

 cipal types, and photomicrographs illustrate the textures of three of 

 the analyzed rocks. 



These dike rocks are of types which are not commom in Colorado nor, 

 indeed, in any part of the world. The full significance of their interest- 

 ing characters can not be determined until the great series of dikes about 

 the Spanish Peaks has been more thoroughly studied. 



The analyzed rocks of the Apishapa quadrangle illustrate very forcibly 

 the fact that magmas of the same chemical composition may produce 

 rocks of notably different mineral composition under the influence of 

 different conditions of consolidation. This is brought out by tables of 

 analyses and norms. The vogesites (orthoclase rocks) are nearly identical 

 chemically with rocks which have been called feldspar basalt, essexite, 

 nephelite basanite, trachydolerite, etc. The incongruities of a purely 

 mineralogical classification of such rocks are discussed. W. C. 



