404 



MINERALOGY. 



Amygda- 



loidat 



itructure. 



Geognosy. 2. Amygdaloidal /ruc/r. When vesicular cavities 

 are dispersed through a basis or ground, and appear 

 empty, encrusted, half filled, or completely filled, such 

 a structure is denominated amygdaloidal. 



The rock named amygdaloid, is a principal example 

 of this kind of structure. Its basis approaches more 

 or less to basalt or greenstone; when it is much iron- 

 shot, it becomes harder and more solid. It is alleged, 

 that while the amygdaloidal rock was still soft, bubbles 

 of air were disengaged, which being prevented escaping 

 by the viscosity of the mass, various shaped cavities, 

 often however of an amygdaloidal shape, were formed. 

 Water holding in solution the various minerals met 

 with in these amygclaloids, is alleged to have traversed 

 the rock, penetrated into the empty vesicular cavities, 

 and to have deposited on their walls its mineral contents. 

 Hence it is maintained, that the amygdaloidal portions 

 are of posterior origin to the basis in which they are con- 

 tained. It cannot be questioned, that some amygdaloidal 

 structures have originated in this manner, but many 

 others are certainly of cotemporaneous formation with 

 the rock in which they are contained. The minerals that 

 usually occur in these vesicles, are lithomarge, zeolite, 

 steatite, chalcedony, agate, heavy-spar, and calc-spar. 

 Those filled with agate and chalcedony present many 

 interesting phenomena. 



3. Granular structure. In aggregated rocks, some kinds 

 are formed by the immediate aggregation of different 

 minerals, which are intimately joined together either by 

 the power of cohesion, or by mutual penetration or in- 

 terlacement. These minerals are generally in grains, 

 and may be regarded as imperfect crystals. Granite 

 affords an example of this kind of structure. 



4. Slaty structure. The slaty structure in rocks com- 

 posed of different minerals, differs from the granular 

 in this circumstance, that the constituent parts are flat, 

 having considerable length and breadth, but incon- 

 siderable thickness. Mica slate, which consists of small 

 plates of quartz and small plates of mica placed upon 

 each other, is an example of this kind of structure. 



Double Double Aggregated Structure. 



aggregated 



structure. ; Under this head we include those varieties where 



Granular 

 structure. 



Slaty 

 structure 



two structures occur together, 

 different kinds. 



The following arc the G*gnosy. 



1. Granvlar-slaly structure. Gneiss affords a good ex- , 

 ample of this kind of structure; the granite and felspar * 

 are in grains, and immediately aggregated together, 

 which forms the granular structure ; and these aggrega- 

 tions are generally disposed in plates, which are inter- 

 posed between the plates of mica, and hence the slaty 

 structure. 



2. Slaty-granular structure Is slaty in the small, Siaty-gran- 

 and granular in the great. The only example of this ularstruc- 

 kind of structure is the topaz-rock of Schneckenstein, ture. 



in Voightland, in the electorate of Saxony. It is com- 

 posed of large granular masses, which have a slaty 

 structure; the slates consist of fine granular quartz, 

 thin columnar schorl, and small granular topaz. In 

 the drusy cavities that occur between the large granu- 

 lar concretions, lithomarge, crystals of topaz, and schorl, 

 sometimes occur ; the schorl, however, is but rarely 

 crystallized. 



3. Granular-porphyritic structure. This kind of Granular 

 structure is granular in the small, and porphyritic in porphyritic 

 the great When large and distinct crystals of felspar structure. 

 appear dispersed through a granular base, as is often 



the case with granite, sienite, and greenstone, this kind 

 of structure is formed. 



4. Slaty porphyritic. This kind of structure is slaty Slaty por- 

 in the small, and porphyritic in the great. The basis phyritie. 

 is slaty, and the porphyritic structure is formed by in- 

 terspersed crystalsor grains of minerals different from the 



basis. Mica-slate, when it contains grains or crystals 

 of garnet, is said to have a slaty .porphyritic structure. 



5. Porphyritic and Amygdaloidal, Here two kinds Porphyritic 

 of structure are placed together, not included in each and amvg- 

 other, as in the preceding kinds. It occurs in many d'idai. 

 amygdaloidal and porphyritic stones. When Amyg- 

 daloid contains, besides the eliptical-shaped masses, 



also crystals of hornblende and mica, it is then said to 

 have an amygdaloidal and porphyritic structure ; the 

 amygdaloidal being the predominant. In Bnsalt, on 

 the contrary, where the two kinds of structure some- 

 times occur, the porphyritic is the predominating. 

 Green Porphyry, although rarely, sometimes possesses 

 this kind of double structure. 



Talk of the Structure of Mountain Rocks. 



Structure of 

 Mountain 

 Stones or 

 Rocks, 



A. Compound, . 



V 



B Simple, 



C Sandstone. 

 ''(i.Conglomerated, J Puddingstone. 

 1 Different frag. 

 ' mented stones. 



fa. Double, 



^6. Aggregated, 



f 



fa. Determinate, J 



vi. Single, 



\ Serpentine 

 UlndwerrmnaU, J it Lhne . 



(_ stone. 



I Gneiss, 

 -j Topaz-Rock 

 i Granite. 

 | III. Granular-porphyritic, -\ Sienite. 



( Greenstone. 

 ] Mica-Slate. 

 | Amygdaloid. 

 V. Porphyritic and 

 Amygdaloidal, 



I. Granular-slaty, 

 II. Slaty .granular, 



IV. Slaty-porphyritic, 



f Limestone. 



1 Gypsum. 

 J Serpentine. 

 "S Clay-Slate, 



/ Quartz-Rock. 



V Flinty-Slate. 



II. Aggregated, 



-I Basalt. 



I. With a bask, . - 



(Green^Porphyry. ( Te Porphyry. 

 1 Green Porphyry. 

 1. Porphyritic, J Porphyry Sla*. 

 1 Porphyrii 

 Gypsum. 



J. OTJiujry oiaiv. 



j Porphyritic Basalt. 



Cr 



.S. Amygdaloidal, J Different kindi of 

 I Amygdaloid. 



,, _ f Granite. 



C 1. Granula, I sienhe . 



J (Greenstone. 



'?. Slaty, 



Mica-Slate. 



