132 the crystalline rocks of cecil county 



Non-Feldspathic Eocks. 

 pyroxenite. 



This type of rock, into which the gabbro readily passes by a 

 decrease in the amount of feldspar, is more or less prevalent along 

 the border of the serpentine and also appears as dikes in the gabbro 

 belt. 



The rocks of this type are somewhat coarse-grained aggregates of 

 hypersthene and diallage. The former mineral is reddish brown in 

 the hand specimen and the latter greenish black. These two con- 

 stituents may be present in about equal proportions, or a decrease in 

 the amount of one or the other pyroxene converts the rock into a 

 diallagite or a hypersthenite. 



Neither of these constituents differs in any particular from the 

 hypersthene or diallage of the hypersthene-gabbro and norite. 



Prismatic cleavage and the parting parallel to go P oo (100) are 

 very marked features of the diallage. Magnetite and sometimes 

 scanty feldspar are accessory constituents. 



The pyroxenites readily alter to smaragdite, anthophyllite, tremo- 

 lite, and other fibrous amphiboles. Pyroxenites, partially altered 

 to amphibolites, and amphibolites in which the alteration is complete, 

 are very common along the serpentine border. The alteration may 

 go further, converting the amphibolite into serpentine, and there is 

 every reason to suppose that pyroxenites as well as peridotites are 

 the source of the serpentines. 



The pyroxenite dikes south of Conowingo show alteration to fibrous 

 amphibole. 



Fresh pyroxenite (hypersthenite) may be found in the vicinity of 

 Oakwood, where it forms a facies of the norite, and south of Cono- 

 wingo, where it occurs as a dike. 



Pyroxenite (websterite) is also found one and a half miles west of 

 Sylmar, near the State line. 



