MINERALOGY. 



405 



Structure of Mountain -matsei and of Simla. 



We understand by the structure of mountain masses* 

 those regular divisions which many of them exhibit, 

 through a considerable extent of rock. There are two 

 kinds of this structure ; the stratified structure, or that 

 of strata ; and the seamed structure, or that of distinct 

 concretions. 



Stratified structure. When a mountain, mountain- 

 mass, or bed composed of one species of rock, is divided 

 by means of parallel seams into masses, whose length 

 and breadth are greater than their thickness, or into 

 what may be denominated Tabular Masses, which extend 

 generally through the whole mountain, it is said to be 

 stratified, and the individual masses are termed strata. 

 Of this kind of structure we have instances in Granite, 

 Limestone. Clay slate, and Mica-slate. The seams that 

 separate the strata are named strata-scams, or seams of 

 Gratification. On a general view, we say the globe i* 

 Composed of formations ; formations of bed; beds of 

 strata ; and such strata as are slaty, of layer*, or 

 slates. 



Strata vary very much in their position. The exa- 

 mination of this appearance of strata is of great im- 

 portance to the geognost and mineralogical geographer. 

 In determining the stratification of a rock, we nave 

 to observe, 1. The direction of the strata. 2. Their 

 inclination. 3. Their thickness. 4. The difference* in 

 their direction and inclination. 



The direction is the angle which the stratum make* 

 with the meridian, and is determined by the compass; 

 or viewing the strata an planes), the direction i* that of 

 a horizunul line, drawn on the surface of the plane; 

 therefore, to determine the direction, is to indicate 

 the point of the compass towards which this line is di- 

 rected. It is always at right angles to the dip. 



The inclination is the angle which the .stratum form* 

 with the horizon, anil i- determined l>y the quadrant. 



The dip in the jxiint (' the compass towards which 

 the stratum inclines. If we know the dip, the direc- 

 tion is given because it is always at right angle* to it. 

 Thus, if a stratum dip to the en.t, its direction must 

 be north and south ; if it dip to the north, its direction 

 must be east and west. But we cannot infer the dip 

 from the direction ; because a stratum, whose direction 

 continues the same, may dip in opposite direc 

 tiiii-. a stratum ranging from north to south may dip 

 either to the eaU or to the west. 



Strata are frequently various!/ waved, thus produc- 

 ing changes in their direction; but t In- -e- change* in 

 direction are in general subordinate to the general di- 

 rection. It U therefore .,; iin|i.TUr.f<-, iti making ob- 

 servations with regard to the position of strata, to distin- 

 guish the general direction and inclination, from the 

 jxrrtiaL To effect this, we muit take the results of 

 number of particular observation*, and compare them 

 together ; and those similar angles which are the moat 

 numerous, are to be considered a* expressive of the ge- 

 : nation and direction. It sometimes happens, 

 that this general position has also H* variations j these 

 May also be attended to and noted. An acquaintance 

 with the lhape of a mountain group, will assist us very 

 much in such Investigation*, as it is intimately con- 

 nected with the general dipo-ition of the stratification 

 if the masses of which it is composed. It i* also of 

 importance to know the fall or declivity of a mountain 

 group, a* its direction and inclination are frequently 



conformable, particularly in the older formations, -with Geogncxr. 

 that of the superimposed masse*. S "^"Y""^ 



It is often very difficult to determine, whether the 

 rock we are examining be stratified or not, and when 

 the seams of stratification are hid, to know the direc- 

 tion of the strata. The following observations will be 

 useful in removing some of the difficulties attending 

 such investigations. 



1. Strata are almost always parallel with the slaty 

 structure of the stone. In certain porphyritic gra- 



the crystals of felspar appear to lie parallel with 

 the strata ; the latter character, however, is by no means 

 so decisive as the former. 



2. Strata can only be formed by parallel seams, which 

 have the same direction and extent through the moun- 

 tain mass. Where parallel rents occur in different di- 

 rections in the same species of rock, as in granite, sand- 

 stone, limestone, &c. it is evident that they are to be 

 considered as accidental. 



3. The seams of tabular distinct concretions, which 

 are often of considerable extent, should not be con- 

 founded with strata seams, because their extent is not 

 so considerable ; and in each group of concretions the 

 direction is different. 



4. Where parallel rents have a different direction 

 from the slaty structure of the stone, they are certain- 

 ly accidental. Inattention to this circumstance, has 

 led mineralogists into error. I observed striking in- 

 stance of these rents in a quarry of gneiss, in the Fo- 

 rest of Tharand, in Upper Saxony. The gneiss, at 

 first tight, appeared to be disposed in vertical strata, 

 and as such it was viewed by DC Luc : on a closer ex- 

 amination, however, the apparent vertical seams proved 

 to be merely accidental parallel rents, perpendicular 

 to the slaty structure of the stone ; therefore the strata 

 were horizontal, not vertical. 



5. Beds are generally parallel with the strata ; these, 

 therefore, point out the direction of the strata. 



6. Although the slaty structure points out to us the 

 direction which the strata must have, it does not fol- 

 low, that a rock having a slaty structure is stratified. 



7. In sandstone, limestone, and rock-salt, regular 

 and very extensive stripes are sometimes observed, 

 which have been confounded with true seams of stra- 

 tification. An attentive examination, however, always 

 discovers them traversing the real strata seams. Von 

 Buch, in his description of Landcdc, and geognostical 

 observations made in Italy and Germany ; and Friesle- 

 ben, in his observation* on Thuringia, describes strik- 

 ing instances of stripes resembling strata-seams. 



Strata vary very considerably in the angle which 

 they form with the horizon ; they occur from hori- 

 zontal to vertical, but the general inclnaition is be- 

 tween horizontal and 45*. The differences at> 

 ther original, or have been produced by subsequent 

 changes. 



Certain species of rock are constantly stratified, while 

 others, either ore not stratified or only in an imper- 

 fect manner. All the secondary rocks, such as lime- 

 stone, sandstone, coal, &c. are very distinctly stra- 

 tified. In the most modem of the secondary rocks, the 

 strata are often horizontal, and frequently extend to a 

 great distance of the same thickness. In more ancient 

 rocks of this class, as in certain limestones and in gyp- 

 sum, toe stratification is less distinct, and the position 

 of the strata is not so horizontal. In the coal forma- 

 tion, which i* of a still older date, the stratification is 

 very distinct, but the bed* are frequently variously 



