122 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



It is difficult to separate in the field the quartz-hornblende-gabbro 

 and the quartz-hornblende-monzonite. The separation is based en- 

 tirely on the character of the feldspar, which can only be ascertained 

 through optical or chemical determinations. The boundary given 

 upon the map is believed to be approximately correct. 



Hornblende-gabbro which usually, and perhaps always, represents 

 the alteration of a pyroxene gabbro (meta-gabbro), occurs in the 

 quartz-hornblende-gabbro belt, in all the dikes from the gabbro body 

 and wherever the norite or hypersthene has been subjected to 

 pressure. 



The quartz-hornblende-gabbros and the hornblende-gabbros are of 

 about the same grain as the qnartz-monzonites, but the contrasting 

 dark green hornblende and opaque white or light grey feldspar pro- 

 duce a more conspicuously mottled effect in the former types than is 

 characteristic of the latter. Feldspar, hornblende and, in a large 

 proportion of these rocks, quartz are the essential constituents and are 

 readily distinguished in the hand specimen. 



The usual accessories magnetite, apatite, zircon and titanite are 

 present. Epidote and chlorite are abundant as alteration products. 



The feldspar of the hornblende-gabbro is, for the most part, by- 

 townite. Orthoclase is sometimes altogether absent and sometimes 

 scantily present. 



Dr. Leonard's determinations, based on stauroscopic measurement 

 of extinction angles on 001 and 010, give approximately the compo- 

 sition of Ab]_ An 4 to the bytownite. That, however, a bytownite 

 with the composition Ab x An 6 is also present, is indicated by some of 

 the writer's measurements. Specific gravity determinations, by 

 means of the Thoulet solution, by Dr. Leonard, gave 2.71 and 2.72 

 for fresh fragments of feldspar. 



The feldspar of the quartz-hornblende-biotite-gabbro is evidently 

 a more acid variety, a labradorite of the composition Abx Anx. 

 Optical determinations and analyses both indicate this to be the case. 



The very general alteration which the feldspar has undergone is an 

 epidotization and a zoisitization. The epidote is singularly free from 

 the greenish yellow tone which usually marks it, and the close aggre- 



