406 



MINERALOGY. 



Geognosy, waved ami convoluted, and often highly inclined. In 

 s ~ 1 "V^"' the transition and primitive classes, many rocks, such an 

 gneiss, mica-slate, and clay-slate, are distinctly stratifi- 

 ed, while others, as granite, porphyry, trap, limestone, 

 gypsum, serpentine, and quartz, are in general but im- 

 perfectly stratified, and the strata of these two classes 

 more often incline to the vertical than to the horizontal 

 position. 



Rocks which appear to have been mere deposites of 

 mud, gravel, sand, &c. have been formed by succes- 

 sive depositions, while others, such as gypsum, lime- 

 stone, quartz, gneiss, mica-slate, clay-slate, granite, &c. 

 owe their stratified structure to crystallization, from a 

 more or less perfect state of solution. This latter opi- 

 nion is illustrated by the uninterrupted transitions of 

 the beds into each other, and by the position of the 

 embedded crystals and masses to the surrounding 

 rock. 



On examining the structure of a mountain or tract 

 of country, we must be careful to avoid all unnecessary 

 minuteness, otherwise we shall fail in acquiring a dis- 

 tinct conception of it. This observation is founded on 

 a knowledge of the geognostical features of nature, 

 which are all on the great scale. 



By not attending to this mode of examination, geolo- 

 gists have fallen into errors, and given to extensive tracts 

 of country a most irregular and confused structure. Spe- 

 culators, building on these errors, have represented the 

 vhole crust of the globe as an irregular and unseemly 

 Hiass. It is indeed surprising, that men possessed of 

 any knowledge of the beautiful harmony that prevails 

 in the structure of organic beings, could for a moment 

 believe it possible, that the great fabric of the globe it- 

 self, that magnificent display of Omnipotence, 

 should be destitute of all regularity in its structure, and 

 be nothing more than a heap of ruins. 



Seamed Structure. Those rocks in which parallel 

 seams occur, that do not extend throughout the whole 

 bed, are said to have the seamed structure. The fol- 

 lowing are the different kinds of this structure: 1. 

 Polyedral. 2. Lamellar. 3. Globular. 



Polycdcal. 1. Polyedral. In this kind of structure, the rock is 

 divided into columns. The columns are sometimes re- 

 gular, sometimes approach to the globular form, and 

 occur even curvated. They are from a few inclies to 

 many fathoms in length. In the islands of Staffa and 

 Eigg, and on the Craig of Ailsa, there are admirable ex- 

 amples of this kind of structure. These columns are 

 sometimes collected into groups, and such groups are 

 often separated from each other by seams or rather 

 vents, which render them more distinct. Such groups 

 may be considered as immense distinct concretions. 

 The columns of such a group often tend towards a 

 centre, others are parallel or perpendicular ; some are 

 horizontal ; and all this variety sometimes occurs in the 

 same hill. The columns are sometimes jointed, so that 

 the convex extremity of the one column is fitted to the 

 Concave extremity of the other ; and these columns are 

 usually composed of globular distinct concretions. 

 These globular concretions are composed of curved la- 

 mellar concretions. The spaces between the different 

 globular concretions are composed of a looser matter 

 than the concretions themselves ; and it is by the fall- 

 ing out of this less compact substance that the struc- 

 ture of such columns is first developed. 



No rock shews this kind of structure more distinctly 

 than basalt : in it we have all the varieties of the seam- 

 ed structure, from the smallest, which is the lamellar 



distinct concretion, to the largest, which is formed by Geognosy. 

 the grouping of columns. -y~^ 



This kind of structure occurs also in porphyry, 

 greenstone, and lava. 



2. Tabular. In this kind of structure, the rock is Tabular, 

 divided into tables a few inches thick, and of small ex- 

 tent. These tables frequently change their direction, 



and then the bed of rock is divided into many groups 

 or systems of tables, having in each group a different 

 direction. 



Their inconsiderable length, frequent change of di- 

 rection, and the even surface of the planes, distinguish 

 them from strata. Basalt, in the lower parts of an in- 

 dividual deposition, has often this kind of structure.. 

 At first sight it is not unlike stratification. It also oc- 

 curs in columnar porphyry. 



3. Globular. In this kind of structure the rock is ei- Globular, 

 ther wholly or partially disposed, or arranged into 

 globes or balls of various magnitudes. These balls are 



either simply granular, as in granite, syenite, green- 

 stone, or sandstone ; granular and lamellar, as in the 

 syenite of Corsica ; compact as in basalt ; porphyri- 

 tic as in porphyry ; or radiated as in limestone. Glo- 

 bular formations are sometimes met with in rocks and 

 minerals having a less crystalline aspect. As examples, 

 we may mention ba'ls of hornstone in limestone, and 

 masses of flint in chalk. 



The roundish balls of granite found dispersed over 

 low countries, have been considered as bowlder or 

 rolled stones, and many theories formed to account for 

 their transportation. The granite of the island of Ar- 

 ran presents this kind of structure. 



Structure of Formations. 



By a Hock forma/ion, we understand a determi- Structure 

 riate assemblage of similar or dissimilar beds or rpck f forma- 

 masses, which are characterised by external and inter- llons - 

 nal relations as an independent whole ; th.it is, as an 

 unity in the series of rock formations. These masses 

 are either simple or compound. When the mass is uni- 

 form throughout, as is the case with limestone or sand- 

 stone, it constitutes what may be denominated a simple 

 formation. Granite, gneiss, and mica-slate, are also of 

 this kind. When dissimilar masses occur in a forma- 

 tion, as in the case of coal and secondary trap, it is said 

 to be a compound formation. 



Similar rocks are often repeated in very different pe- 

 riods. Each of these individual depositions is a parti- 

 cular formation, and the whole is denominated a scries 

 or suit of formations. Thus there is a limestone, a por- 

 phyry, a granite suit, &c. 



It is, Werner remarks, a determinate character of 

 certain formations, to constitute the principal mass of 

 the mountain in which they occur : this is the case with 

 gneiss, clay-slate, porphyry, and others. With other 

 formations, on the contrary, it is as essential a character 

 to occur only in single beds in the others, and these are 

 said to be imbedded. Primitive porphyry, limestone, and 

 many others, are of this kind. When such individual 

 beds occur in different principal formations, (that is, are 

 not confined to a single one, ) as primitive limestone and 

 primitive trap in gneiss, mica-slate, and clay-slate, and 

 always continue the same, notwithstanding the differ- 

 ence of the rocks in which they are embedded, they 

 are to be considered as independent formations. If, on 

 the contrary, they are confined to one formation, and 

 are connected with no series or suite of formations, 

 they are said to be subordinate to it. Alum slate and 



