M I N E R A L O G Y. 



403 



. them completely enjoyed, by an intimate acquaintance 

 with nature herself', not in cabinets alone, hut in mines 

 and amonp mountains. On descending into mines, we 

 are not only gratified by displays of human ingenuity 

 and skill, but we also receive much information respect- 

 ing the structure of the earth, and the it ha* 



experienced during the different period* of its forma- 

 tion. Our researches on the surface of the earth, on 

 the other hand, often lead us among the grandest and 

 mot sublime works of nature ; and amid alpinr group*. 

 oologiit is, as it were, conducted nearer to the 

 tcene of those great operation*, which it is hi. business 

 to explore midst of such scene*, he feels 1m 



mind invigorated ; the magnitude of the appearances 

 before him extinguishes all the little and contracted no- 

 tions he i formed in the closet ; and he learns, 



that it is only by visiting and studying these stupendous 

 works, that he can form an adequate conception of the 

 great relations of the crust of the globe, and of its mode 



Nature appear*, in her formations, frequently to pro- 

 ceed from the moat special to the mart general ; or, we 

 observe the most general, including! series of gradually 

 diminishing subordinate differences. In simple mineral*, 

 many instances of this occur, M in the dittinct concre- 

 tions, fracture, and external surface. Hut it i- more -tak- 

 ingly the case in the structure of the crust of the earth , for 

 here there is a teriesfromthatoflhcsimple mountain rock 

 to those grand and extensive arrangements in the forma- 

 tion* that form whole mountain* and chains of moun- 

 tain*. 



Four different kinds of structure are observable in the 

 crust of the earth. The Jint is, that which i* to be 

 observed in hand specimens ; it i* the smallest kind of 

 structure, and occur* in what are termed Mountain 

 Hockt or Stone* The second kind of structure, or that 

 of Mountain Matin, i* more on the great scale, and i* 

 not to be observed in hand specimen*, but only in sin- 

 gle ina*s*i of rock. To this structure belong* stratifi- 

 cation, and the learnt of distinct concretion*. The third 

 kind of structure is that of Hack FormMtioni, or 

 peat ma**** of which the crust of the earth i* 

 To examine thi* kind of structure, we must traverse 

 consiilcr.il >le tract* of country. The fourth kind of 

 structure, i* that of the Earth itself, which i* formed 

 by the junction of various formations. To examine this 

 structure, we must travel through many countries. 



We have thus a geognostic aerie* of structure from 

 hand specimens, which can he examined in the closet, 

 to that which can only be known by travelling through 

 many countries. 



We shall now describe the different kinds of struc- 

 ture, in the order already mentioned. 



Structure of Mountain Rockt. 



Structure Mountain Rocks or Stone*, are those mineral masses 

 of moon- O f wn j cn the greater portion of the crust of the earth 

 tain rueki , 



u composed. Mineral*, or mineral aggregate*, to 

 have the true character* of mountain rocks, must 

 occur not only in great masses, but frequent I v, and 

 present in their structure and composition such cha- 

 racter* as will serve to distinguh them, and make them 

 known in whatever situation they may be found. 



They are either -unple, or aggregations of simple 

 minerals. In the one case, they are denominated 

 Sim/.lf mountain rufii , in the oilier. Compound mountain 



Simple mountain rocks are either compact, slaty, or Geognosy 

 granular. Clay-slate, limestone, and serpentine are ex- '^^^ 

 arrptcs of simple mountain rock*. untain 



When minerals occur disseminated through simple ,. oc i is _ 

 mountain rocks, they are to be considered as accidental, 

 and <1 > not entitle us to consider such varieties as dis- 

 tinct species of rock. Thus garnets imbedded in gra- 

 nite or mica-slate are accidental mixed parts. 



('.impound mountain rocks are either eonglomera- Compound 

 ted or azresated. This distinction is founded on mountain 

 the mode of their formation. The grains or masses in the lockt - 

 conglomerated mass, according to some naturalists, have 

 not been formed on the spot where they are now found, 

 but have been carried thither, and connected together 

 by a cement. To this class belongs sandstone, pudding* 

 stone, and different kinds of fragmented or brcccialed 

 stones*. 



In aagrtg ateil mountain rocks or stones, their present 

 structure is their original one, and the parts of which 

 they are composed have been formed on the spot where 

 we now find them. Their parts are immediately con- 

 nected together ; hence this structure i* termed aggre- 

 gated. 



Aggregated mountain rocks are either determinate- 

 ly or iiuittrrminattlii aggregated. The parts in the 

 imlflerminatelu aggregated, are irregularly and con- 

 fusedly joined together. This kind of structure occurs 

 in those varieties of serpentine, where limestone and 

 serpentine are *o conjoined that it is difficult to *ay 

 which predominates, and where the one sometimes en- 

 close* the other. The Campan marble, which is an ir- 

 regular mixture of limestone and steatite, and many 

 Cipolin marbles, which contain veins and patche* of 

 green talc, are of the same description. 



The detmninalrly aggregated ttruclurr present* a 

 number of subordinate difference*. It is either timple 

 or double aggregated. The timple aggregated contains 

 two subordinate kinds. In thejf>*, the minerals are 

 connected together in such a manner that one serve* a* 

 bacw for the other, which is included in it ; and it 

 also contain* two subordinate kinds. These are deno- 

 minated the porplii/ritic and amyndatoidal. In the te- 

 cond, all the psrts are immediately connected, or joined 

 together ; and here we have also two subordinate kinds, 

 the granular and ilaiy* 



The double aggregated includes five subordinate kinds : 

 The*e are, 1. Granular tlatu. 2. Slaty granular. S. 

 Ciraimlar porphyriiic. 4. Slaty porphyrilic ; and, 5. 

 Porphyritic and amifgdaloidal. The first four kinds of 

 double aggregated 'structure comprehend one structure 

 in another, so that, as the denominations intimate, a 

 smaller structure is contained in a greater. In the fifth, 

 or last kind, one docs not include the other; but, as 

 the denomination expresses it,- they are placed near or 

 beside each other. 



We shall now describe each kind in particular. 



Simple Aggregated Structure. 



1. Porplii/ritic tincture When one of the constitu- Simple ag. 

 ent parts of the mountain rock i* disseminated through *** 

 a basis, in the form of grains or crystal*, the rock pre- *' 

 senting this appearance is said to be j^r/ ; liyritic. Com- 

 mon porphyry, porphyry slate, and gypsum contain, 

 ing crystals of quartz, may be mentioned as examples 

 of this kind of structure. The crystals or grains are 

 here of cotemporancous formation with the basis, and 

 not mechanically mixed, as some have maintained. 



We thall afterward! examine particularly the ab*t, which U ihr general opinion, in rrgsrd to the formation of conglomerated rocks. 



