IGNEOUS ROCKS 315 



I. Quartz-biotite-hornblende-gabbro. Near the foundry on Stone run, Cecil 

 county, Maryland. 



II. Quartz-biotite hornblende-gabbro. Near Porters bridge, Octoraro creek, 

 Cecil county, Maryland. 



III. Gabbro from the neighborhood of Radnor and Bryn Mawr. Analysis made 

 by W. F. Hillebrand from three specimens representing three localities. 



IV. Pyroxenite (Websterite). Oakwood, Cecil county, Maryland. 



V. Norite. Near McKinseys mill, Cecil county, Maryland. 



VI. Hornblende-gabbro. Stone run, seven-eighths of a mile northwest of Ris- 

 ing Sun. 



Analyses II-VE were made by W. F. Hillebrand for Dr A. G. Leonard. Journal 

 of American Geology, volume xxviii. pages 170-171. 



Numbers I and II fall into class II, order 4, rang 4, and subrang 3. This means 

 that the salic constituents are dominant over the femic constituents. Of the salic 

 constituents feldspar is dominant over quartz, and lime-soda feldspar with dom- 

 inant lime molecule is the prevailing feldspar. 



Of the femic constituents the silicates are predominant and premirlic. 



The structure of these rock species is the hypautomorphic granular ; the abnor- 

 mal ve minerals are biotite and hornblende in approximately the proportion of 

 1 : 2. These two species are therefore biotite-hornblende-grano-bandoses. 



Number III falls into class II, order 5, rang 4, subrang 3, and is a dosalane 

 hessose. 



As in the case of the bandose, the salic constituents are dominant and femic 

 constituents subordinate. Of the salic constituents feldspar predominates to an 

 extreme degree, a departure from the bandose type, and lime-soda feldspar with 

 dominant lime is the prevailing feldspar. Augite is the only abnormative mineral 

 among the essential constituents of the rock. This is frequently present in notable 

 amount and then plays the role of a critical mineral. The structure of the rock is 

 hypautomorphic granular, and it may therefore be called an augitic grano-hessose. 



Number IV is a diallagic-grano-cecilose (class V, order 1, rang 1, section 2, sub- 

 rang 2) — that is, the rock is extremely rich in femic silicates, which are permirlic, 

 permiric, and domagnesic in character. The salic constituents in this species are 

 negligible. 



Number V is a grano-kedabekose (class III, order 5, rang 5, subrang 3). The 

 absence of mineral prefix in the name indicates that the rock contains no abnor- 

 mative minerals. Salic and femic constituents are present in about equal propor- 

 tions ; of the salic constituents feldspar, and that a lime-soda feldspar, is dominant. 



Number VI approaches I and II closely, and differs from them only in being 

 percalcic. It is a biotite- hornblende-grano-corsose (class II, order 5, rang 5, sub- 

 rang 3). 



Among the gabbroitic intrusives there are then four dosalanes, one salfemane, 

 and one perfemane. The dofemanes are doubtless represented by some of the 

 peridotites of which analyses have not been secured. The granitic intrusive rep- 

 resents the persalanes as well as the dosalanes. 



All of the five classes of igneous rocks are thus represented among the 

 intrusives of the Pennsylvania Piedmont. With the exception of the 

 non-feldspathic species ( IV), the intrusives are either dofelic or perfelic. 

 They are all, moreover, either docalcic or percalcic and presodic. While 



