MARYLAND GEOLOGICAL SURVEY 115 



and muscovite, The former occurs in shreds or blades of a green 

 color, which usually show a parallel grouping. It is sometimes re- 

 placed by chlorite. The latter mica occurs both as blades and in 

 aggregates of microscopic scales, forming bands in a mosaic of inter- 

 locking quartz grains. In this fine-grained quartz mosaic are larger, 

 rounded quartzes giving a clastic appearance to the section. 



Staurolite, garnet, tourmaline, zircon, apatite, rutile and magnetite 

 are accessory constituents. Staurolite crystals may be found in the 

 field most abundantly where Basin Run has eroded the gneiss. They 

 show, under the microscope, a complete change to an aggregate of 

 microscopic muscovite scales. This is a not unusual alteration of 

 staurolite and has been described elsewhere. 



The garnets are often partially or completely altered to an aggre- 

 gate of chlorite scales. The alteration begins at the periphery and 

 proceeds towards the interior. A central garnet core, therefore, 

 remains where the alteration is still incomplete. The chlorite usually 

 preserves the crystal form of the garnet, but this may be somewhat 

 obscured by the growth of chlorite into the surrounding matrix. 

 Garnet, tourmaline, and magnetite occur both in the gneiss and in 

 the altered staurolite. Zircon and apatite are included in the quartz 

 of the gneiss. Rutile needles occur in the chlorite. This mineral 

 is often a conspicuous secondary constituent. Epidote is rarely pres- 

 ent as an alteration product, 



Analyses I and III, given below, are from this formation, as it 

 occurs to the north in Pennsylvania and represent the general com- 

 position of the mica-gneiss found in Cecil county. There have been 

 tabulated with them an analysis of a biotite-gneiss from Freiberg, 

 Saxony, two analyses of Canadian gneisses, and two analyses of slates, 

 with which the gneisses may be compared. 



That, in general, the gneisses correspond in composition to the 

 slates is plain. Analysis I represents a more silicious slate. The 

 characteristics which these analyses have in common are the high 

 percentage of alumina, the low percentage of the alkalies, and the 

 preponderance of magnesia over lime. These are characteristics 

 common to silicious argillites. 



