398 IDIOPHYSICS. 



a certain ridge, the strata dip or decline in opposite directions, that 

 ridge is called an anticlinal line or axis : and when the strata decline, 

 on both sides, towards a line of meeting, this latter is called a syncli- 

 nal line or axis. Strata which are twisted, or bent, are said to be 

 contorted : and they are said to be conformable, when their surfaces 

 are nearly parallel ; but unconformable, when this is not the case. 



When the strata present their edges so as to be visible at the sur- 

 face, they are said to crop out: and when their outcrop is abrupt or 

 precipitous, it is called an escarpment. Detached portions of strata, 

 which remain elevated, while the surrounding parts have been re- 

 moved, are called outliers. A stratum of small extent, is called a 

 bed; and a very thin stratum is sometimes called a seam. When 

 the strata have been apparently broken across, and those on one side 

 raised above those on the other, so as to break the continuity, this 

 break is called a fault. When some volcanic or other matter has 

 been injected into the break, so as to form a wall or partition crossing 

 the strata, this wall is called a dyke ; or, if small, a vein ; such as 

 often contain metallic ores. A series of rocks supposed to have been 

 produced at about the same time, is called a formation. 



The lowest rocks which have been examined by geologists, are 

 found to contain no organic remains, and no coal nor salt : but they bear 

 the marks of igneous origin, and are supposed to have crystallized 

 in cooling from a melted state, before the higher strata were formed, 

 or any animals existed. Hence they have been called primary 

 rocks ; including granite, sienite, and similar rocks, which are the 

 lowest, and unstratified ; and gneiss, mica slate, and primary lime- 

 stone or marble, which are all stratified, and crystalline, when they 

 belong to this formation. These rocks extend beneath the lowest 

 valleys ; but protrude above them, and form the masses and tops of 

 the highest mountains ; towards which the strata incline upwards, as 

 if the mountains were upraised by a subterranean force. Talcose 

 and chloritic slates, and quartz rock, occupy extensive areas of the 

 earth's surface ; but their crystallization is less distinct than in those 

 before mentioned ; and these rocks are, in many places metamorphic, 

 or more or less altered by heat, from injected igneous rocks, as trap 

 dykes, or granite, sienite, and quartz veins. 



The rocks next above the primary, have been called transition 

 rocks ; including the Cambrian and Silurian systems : and they 

 contain occasional shells, as the ammonite, (Plate IX. Fig. 1.), the 

 belemnite, (Fig. 2.),* the orthoceratite, (Fig. 3.), and the trilobite, 

 (Fig. 4.) ; some fishes, as the orodus, of the shark family, (Fig. 5.) ; 

 with some zoophytes, marine plants, algae, and ferns, but no organic 

 remains, of a higher class, and but slight traces of coal or salt. They 

 include argillite or clay slate, gray wacke, granular limestone, gypsum, 

 and sometimes granite: but the granite, and similar rocks, which 

 occasionally overlie the slate or limestone, were probably ejected 

 from below, and deposited, like lava from volcanoes, long after the 

 primary formations. Transition strata often extend up the sides of 

 primary mountains ; and sometimes constitute extensive mountain 



* The belemnite appears to have been the internal shell or skeleton of an animal 

 resembling the cuttle-fish, hence called, by Buckland, the belemnosepia. 



