144 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



Each of these types carries one or more dark-colored minerals and 

 sometimes other light-colored minerals and the minor subdivisions are 

 based on which of these are present. Sometimes the name of the mineral 

 is attached to the family name as, for example, hornblende granite; or at 

 other times a new name is used, such as norite, which is applied to a 

 member of the gabbro family in which the most important dark-colored 

 mineral is hypersthene. 



The Sedimentary rocks are composed almost entirely of deposits formed 

 under water by the accumulation of rock or mineral fragments, by or- 

 ganic agency or by chemical action. The rocks differ widely in compo- 

 sition and character and do not allow as strict a classification as do the 

 igneous rocks. They may be divided first according to their composition 

 into: — 



Arenaceous, or more or less coarse-grained sandy or pebbly deposits; 



Argillaceous, or fine-grained, clay-like or earthy deposits; 



Calcareous, or composed essentially of carbonate of lime, to which might 

 be added a fourth; 



Pyroclastic deposits, composed primarily of volcanic dust and ashes. 



According to their coarseness and the shape of their constituents may 

 be distinguished: 



Among the first: sands and sandstones, gravels, grits and conglom- 

 erates. 



Among the second: clays, mudstones, shales and clay-slates. 



Among the third: marls, greensands and limestones. 



Among the fourth: volcanic dust, tuffs, breccias and conglomerates. 



Almost all of the sedimentary rocks show a stratified arrangement of 

 their constituents which generally lie in approximately parallel planes. 



The Metamorphic rocks are formed from either sedimentary or igneous 

 rocks when the latter are changed in composition and texture by the 

 recrystallization that g'oes on when rocks are subjected to pressure or 

 heat in the earth-movements which are constantly taking place. The 

 metamorphic rocks are frequently rendered platy or schistose by the 

 recrystallization under pressure, and they are accordingly often de- 

 scribed as schists. Just as their origin is more complicated than that 

 of the igneous or sedimentary rocks, so their classification is less thor- 

 oughly worked out. If the character of the original rock is known to 

 be some type of igneous rock it is customary to prefix the term meta to 

 the name of the original rock. Thus, meta-gabbro is applied to meta- 

 morphic rock which is known to have been originally a gabbro. 



Some of the more technical words used in the descriptions of the rocks 

 may be defined as follows: 



Acidity. — A term used to indicate the amount of acid, usually silicic 

 acid, compared with the amount of metals, such as iron and magnesium. 

 Igneous rocks usually grow lighter in weight and color with increasing 

 acidity. 



Actinolite. — A fibrous green silicate of lime, magnesium and iron. Usu- 

 ally the product of metamorphic changes in rocks. 



