76 STRUCTURAL GEOLOGY. 



To appreciate tlie nature and qualities of fragmental material, the student 

 should go to the hills where sand and gravel are dug ; to the sea-beaches 

 where the waves are at their grinding and assorting work, or to the estuaries 

 where mud-flats and sand-flats have been made by their greater action ; to 

 the river-valleys, where plunging streams are at their abrading and destroy- 

 ing work, or where quieter streams are bordered by terraces of sand, or 

 gravel, or loam, or clay. All is fragmental material ; and all these results 

 of attrition and partial decomposition may be included under the four 

 divisions of (1) sand, (2) gravel (or a mixture of stones and sand), (3) earth 

 or mud (according as it is wet or not), and (4) clay. The last, the material 

 of brick and pottery, is plastic when wet, and feels a little greasy. Mud of 

 the finest kind is usually more or less pure clay. 



Fragmental deposits are made up of successive beds or layers ; that is, 

 are stratified (using a term from the Latin stratum, a bed). They are also, 

 for the most part, sedimentary beds, the sand and earth deposited by water 

 being its sediment; and hence they are often called sediments. The waters 

 that deposited the sediment and made the stratified accumiilations were 

 mostly those of the ocean, or of rivers or lakes ; and sea-border, fluvial, and 

 lacustrine formations are illustrations therefore of fragmental deposits. 



Crystalline Eocks. — Nearly all substances crystallize on passing to 

 the solid state from a previous state of either fusion, solution, or vapor, and 

 many without fusion if subjected to long-continued heat. The grains of a 

 massive crystalline rock are, in the main, or wholly, imperfect crystals. They 

 are generally angular in form ; and when so, it is usually because of the 

 cleavages of the constituent mineral grains. Being crowded together, they 

 very seldom have the external planes of crystals. Granite and crystalline 

 limestone (or ordinary white marble) are examples. In crystalline lime- 

 stone, all the grains are angular and glisten, owing to the cleavage-surfaces. 

 In granite, those of two of the constituent minerals show sparkling cleavage- 

 surfaces, but the third, quartz, is without cleavage. When the grains are 

 distinctly visible without a glass, the texture is described as macroscopic; if 

 undistinguishable, the texture is microscopic, or aphanitic. 



Crystalline rocks are, to a large extent, igneous or eruptive rocks ; that is, 

 they have become crystalline masses from a state of fusion, as, for example, 

 lavas and the many kinds of igneous rocks. Others have become crystalline 

 by heat without fusion, with or without attending change in composition ; for 

 example, a massive limestone has thus been changed by simply long-continued 

 heat to a crystalline limestone or marble, granitic sandstone to granite or 

 gneiss, and so on. Such rocks are called metamorpJdc rocks. Fragmental 

 rocks have been thus metamorphosed on a large scale during times of moun- 

 tain-making. Metamorphic rocks have sometimes been subjected to a second 

 partial or complete metamorphism, and igneous rocks occur altered in like 

 manner. Crystalline rocks are usually mere mixtures, like the fragmental, 

 as they consist of one, two, or more minerals in various proportions. If of 



