MARYLAND GEOLOGICAL SURVEY 



143 



The youngest crystalline formations of Cecil county are sundry 

 small diabase dikes of Triassic age. These resemble in every 

 particular the diabase of the Triassic, which forms notable high- 

 lands in the northeast, where it has been thoroughly investigated and 

 described. 



GLOSSAEY OF GEOLOGICAL TEEMS. 



The rocks found at the surface of the earth have been formed in one 

 of three ways. They are the product of consolidation from a heated 

 molten state; they are the accumulation of fragments of disintegrated 

 rock, which have been carried mechanically by running water and me- 

 chanically deposited or have been carried in solution by running water 

 and deposited by means of chemical precipitation or through the agency 

 of organisms; or they are modified representatives of members of the 

 first or second class which have been changed or metamorphosed by forces 

 which have acted upon them. Eocks of the first class are called Igneous; 

 those of the second, Sedimentary; and those of the third, Metamorphic. 



The Igneous rocks are subdivided according to their chemical composi- 

 tion, more or less perfectly expressed by the minerals of which they are 

 composed; and their texture, or manner in which the mineral constitu- 

 ents are distributed in the rock. 



Since the species of the common light-colored mineral feldspar, or the 

 closely related feldspathoids, varies with the chemical composition of the 

 rock, it is customary to divide the Igneous rocks into the several divis- 

 ions which, with the few additional subdivisions rendered necessary by 

 the abundance of certain types, form the different families of igneous 

 rocks. 



Each of these families defined by its chemical and mineralogical com- 

 position is subdivided according to the differences in texture which de- 

 pend upon the circumstances under which the rock consolidated and 

 which often indicate the relative distance beneath the surface of the 

 earth at which a given rock was formed. 



A combination of these various factors of chemical and mineralogical 

 composition and texture applied to the rocks found in Maryland leads to 

 the following: 



Classification of Igneous Rocks found in Maryland. 



Potash 

 feldspar. 



Granular. Granite, 

 texture. 



Fine grained 

 granular or 

 ophitic 

 texture. 



Porplryritic, Rhyolite 

 microgranular 

 or glassy 

 texture. 



Soda-lime feldspar. 



Monzonite. 



Diorite. 



Lime-soda 

 feldspar. 



Gabbro. 



Norite. 



Diabase. 



Basalt. 



No feldspar. 



Pyroxenite and 



Peridotite. 



