Maryland Geological Survey 125 



B. Under the Microscope 

 7. Very Fine Sand 



(1) Light 

 Ratio of quartz to feldspar estimated 90 : 10. 1 



With the light portion there is separated an abundance of grains of a translucent to 

 opaque, humus-brown substance full of small dark granules. The substance is isotropic, 

 index of refraction 1.55-1.56. It crushes plastically under the knife. Probably it is a 

 combination of organic and inorganic colloidal matter, with inclusions of granules that 

 may be both mineral and carbonaceous but are not fresh mineral grains. 



(2) Heavy 

 Dominant. — Glauconite in worn grains ; percentage as given. 



Abundant. — Magnetite, garnet, epidote, muscovite, pyrite in granular concretions. 

 Barer. — Tourmaline, staurolite, chlorite, biotite, topaz, rutile, zoisite, zircon, enstatite, 

 kyanite, anatase (dumortierite?). 



The well-rounded form of the magnetite grains is noteworthy. 



77. Finer Portions 

 The finer portions (extra fine, silt, and clay) show little of special interest. The 

 clay is gray with a strong humus-brown stain, and contains unusually much of a dirty 

 fibrous matter that is common in many of the samples. 



Summary and Conclusions. — Noteworthy are : 



( 1 ) The abundance and variety of heavy minerals. 



(2) The high percentage of magnetite with associated garnet and 

 epidote. 



(3) The fact that the glauconite is all rounded, i. e., reworked. 



(4) The rounded clay-like grains. These may be merely undisinte- 

 grated clay, though their abundance would seem to indicate some 

 concretionary process, perhaps the first stages in the formation of glau- 

 conite, as will be explained in the general discussion of glauconite (see 

 p. 176 below). The abundance of pyrite in the sample, however, suggests 

 that pyrite may have something to do with the formation of these granules, 

 though I believe such a process has not hitherto been recognized. 



(5) The pyrite concretions. Pyrite concretions are, under certain 

 conditions, formed in waters in which abundant organic matter is 

 decomposing. 



(6) The lack of sorting indicated by the abundance of several different 

 sizes of sand and the high percentage of magnetite and garnet. 



1 The ratio of quartz to feldspar was determined by making several counts, in different 

 parts of the slide, of all the grains in the field of view of a No. 4 objective and deter- 

 mining the number of these that were feldspars. The feldspars were rather readily picked 

 out, following Thoulet, with the aid of a liquid of index 1.548 (the mean index of quartz), 

 checked when necessary by determining the optical figure. The average as will be seen, 

 is always given to the nearest 5 units. The relative sizes of the grains was not con- 

 sidered, so that the results have no absolute quantitative value. They do serve, how- 

 ever, to indicate the relative abundance in different samples. 

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