GEOLOGY. 



399 



species from the mass enclosing them. These rocks 

 are culled amygdaloidal. Many rocks contain acci- 

 dental substances, besides their regular constituent 

 parts ; various sorts pass into eacli other by gradual 

 changes, or there is a change in some of their con- 

 stituent parts. They also undergo various decompo- 

 sitions from the action of water, air, &c. 



Stratification and Divisions of Rocks. In stratifi- 

 cation, we find large masses, and even mountains of 

 rocks, divided, by parallel clefts, or splits, into large 

 and often very extensive parallel masses or strata. 

 These strata differ, in being more or less distinct, 

 regular or irregular, straight or undulating. They 

 are seldom found to be perfectly horizontal. Some 

 species of rocks are found distinctly stratified ; some 

 partially so, and some not at all. Sometimes one 

 stratum rests upon another, and is itself covered by 

 a third. In this case, the second is called the subja- 

 cent, or inferior, and the third the superincumbent, 

 or superior, stratum. The thickness of the strata is 

 very unequal. The extension of strata in a particu- 

 lar line, is termed their direction, and is ascertained 

 by means of the compass. Their deviation from a 

 true level, is called their dip, and is estimated by 

 degrees of a circle, and according to the four cardinal 

 points like the direction and dip of the magnetic 

 needle. The portion of the strata above the surface 

 of the earth, is called their visible end, or extremity. 

 Among the most interesting and important of the 

 phenomena connected with stratification, are the 

 breaks which not unfrequently occur in copper, coal, 

 and other mines, where one rock seems to have 

 slipped by the adjoining one, or to have changed its 

 place, so that the metallic or other vein running 

 through them both, is interrupted, and the continua- 

 tion of it is thrown higher or lower than the first 

 part. These are technically called shifts. Rocks 

 are divided again, according to their more or less 

 regular form, in which respect they are called 

 columnar, tabular, spherical, &c. By the position of 

 rocks is understood their place in the general ar- 

 rangement of the rocky masses which form the ex- 

 ternal covering of the earth. The position of rocks 

 is either conformable, unconformable , or overlying. 

 The position of rocks is said to be conformable, when 

 the edges of the strata of a rock lying upon another, 

 present the same appearance and arrangement as 

 those of the one upon which it rests. It is uncon- 

 formable, when the rocks which lie upon older forma- 

 tions present a different appearance of strata from 

 the other, they being different in dip or direction. 

 The position is overlying, when the strata of the 

 superior rock conceal from the view the position of 

 the rock below. Alternating position is when two 

 or more kinds of rocks lie upon each other in re- 

 peated succession, and thereby indicate a contempo- 

 raneous origin. Parallel formations are when differ- 

 ent rocks alternately take each the place of the 

 other. The particular situation of minerals, their 

 course and position, which constitute the basis of all 

 mining operations, are of great importance. The 

 veins of minerals are the tabular or flat spaces, either 

 in part or entirely filled with different mineral sub- 

 stances, by which masses of rock are intersected, and 

 for the raost part in a direction forming a greater or 

 less angHe with the direction of the strata of rock. 

 These courses or veins of minerals follow straight 

 lines of direction, or they are bent and curved in 

 various directions and forms. The mineral which 

 fills the vein is more or less different from the rock 

 in which it occurs, according to circumstances, or is, 

 at least, distinguishable from it. The direction of 

 the vein is estimated by the angle which it forms 

 with the meridian ; its inclination, by its dip, or the 

 angle it forms with the horizon. Some veins have 



no particular direction or dip, but extern! in al! 

 directions. The rock upon which the vein lies, is 

 called the lying, and the one which covers it, the 

 hanging, rock. The vein terminates at its outgoing 

 upon the surface of the mountain or earth ; the other 

 end tends towards the interior of the earth. The 

 thickness of the vein is estimated by the distance 

 between the underlying and the overhanging rock. 

 A vein is sometimes compressed, or diminishes in 

 thickness ; it sometimes stops in the direction of its 

 length ; and it is said to be lost, when it splits into 

 several small veins. The vein consists either of one or 

 of several species of minerals ; it contains cavities of 

 various form and size, either filled with minerals or 

 having their sides encrusted, or covered by crystals 

 of various kinds, which cavities are called druses. 

 The substance of the vein is sometimes firmly united 

 with the rock adjoining it, and is sometimes separated 

 by clay, earth, &c. The relative position of several 

 beds and veins of mineral substances, in any moun- 

 tain or country, is of great importance in mining. It 

 is seldom that perfect regularity exists among the 

 various mineral deposits in any vicinity ; they more 

 commonly vary in their direction, and thus cross and 

 intersect each other. Very extensive deposits of 

 minerals, of limited length, are termed standing beds, 

 or masses; and mountain masses, intersected by 

 great numbers of small veins and deposits, are called 

 floors. Beds and layers of minerals are particular 

 masses, of a flat or tabular structure, running in the 

 same direction with the strata, but differing from the 

 rocks in which they are contained, in composition 

 and structure, as well as in other circumstances. 

 Foreign deposits, of various kinds, occur in moun- 

 tains, and in rocky districts of all sorts. Their 

 direction and dip are generally the same with those 

 of the mountain masses containing them. Mineral 

 deposits consist either of simple minerals, unmixed, 

 or of rocks. Many deposits contain both. (For an 

 account of petrifications, see Organic Remains.) 

 The substances of which the subjects of these re- 

 markable changes consist, are chiefly calcareous, less 

 frequently siliceous, or combustible minerals ; also 

 ores. The presence of petrifactions, especially in 

 rocks of new or later formation, is a circumstance of 

 great importance in a geological point of view; 

 since, by a careful consideration of them, it has been 

 ascertained, that successive generations or creations 

 of animal species, such as are not now living any 

 where, are found buried in rocks, in such order that 

 similar or related species are found in rocks and 

 situations of a similar character ; and that they differ 

 more or less, according to the antiquity of the rock 

 formations in which they occur. And in this manner 

 a ground is afforded for solid conclusions in regard 

 to the antiquity, or period of formation, of many kinds 

 of rocks. See Plate 40 for a geological map of Europe. 

 Divisions of Time in the Formation of Mountains, 

 and the Classification of Rocks. The circumstances 

 of the relative position of rocks, enable us to form 

 some comparisons between them, in regard to their 

 antiquity, although we are unable to state the expres? 

 period of their respective formations. They are 

 divided, in this respect, into primitive, transition, 

 secondary, orfloetz rocks, alluvion and volcanic rocks; 

 or, according to a more recent division, into^rwnary 

 or primitive, secondary, tertiary, volcanic, diluvial, 

 and alluvial deposits comprehending all rocks and 

 earths. Primitive rocks are crystalline in structure, 

 and are remarkable for the great purity and firm 

 adhesion of their component parts. Siliceous and 



rgillaceous earths form the chief ingredients in their 

 omposition, and they are remarkable for the absence 

 of all petrifactions to testify the previous existence 

 of organic beings. When both classes occur to- 



