432 



MINERALOGY. 



Imbedded 

 minerals. 



Decompo- 

 sition. 



fieognosy. coarser varieties is large, or small-grained uneven; of 

 N ^"V^" / the more crystalline varieties, even inclining to large 

 and flat conchoidal, and seldom to splintery. It is 

 opaque, or feebly translucent on the edges. It yields 

 a pale grey-coloured streak. It is semi-hard, border- 

 ing on hard. It is rather brittle, and is difficultly fran- 

 gible. Specific gravity = 3.08. 



Structure. Structure. It occurs in distinct concretions of vari- 

 ous descriptions. They are generally columnar, vary- 

 ing from a few inches to some fathoms, even to upwards 

 of 100 feet in length ; the number of sides varies from 

 three to nine, and of these the nine-sided are the rarest; 

 they are straight, curved, and either parallel or diverg- 

 ing; sometimes they are articulated, and the joints 

 have concave and convex faces. In mountains, these 

 concretions are collected into large groups, and many 

 of these groups or colossal concretions form a hill or 

 mountain. Sometimes it occurs in tabular, sometimes 

 in globular concretions ; these latter are frequently 

 composed of concentric lamellar concretions, or of co- 

 lumar concretions radiating from a centre. Some va- 

 rieties are composed of large, coarse, and fine granular 

 concretions. 



There is sometimes a tendency to stratification, and 

 varieties occur with the amygilaloidal, and also with 

 the porphyritic structures. 



Imbedded Minerals. The most frequent imbedded 

 minerals are olivine and augite ; besides these, grains 

 and crystals of felspar, and also of basaltic horn- 

 blende, calcareous spar, and magnetic iron ore, are met 

 with. 



Decomposition. Some varieties are easily decom- 

 posed, particularly those that incline to wacke and 

 amygdaloid, while others long resist the action of the 

 atmosphere. The earth formed by their decomposition 

 has a greasy feel, and the great fruitfulness of basalt 

 countries is owing to this basaltic earth. Sometimes 

 the imbedded minerals decompose, at length fall out, 

 and thus leave the basalt with a vesicular structure. 

 Olivine is one of the most easily decomposable of the 

 simple minerals met witli in basalt, being even more 

 easily broken down by the influence of the weather 

 than calcareous spar ; on the contrary, we often find 

 augite and hornblende unchanged, after the basalt has 

 been reduced to a clayey mass. 



Forms. Forms. It occurs frequently in the form of moun- 



tain caps, having tabular or conical forms ; or it is dis. 

 tributed in long ridges, or in scalarlike cliffs and ter- 

 races. 



Geognostic Geognoslic and Geographic Situations. It occurs in 

 and geo- beds, imbedded masses, and veins in sandstone and 

 graphic si- limestone formations in Scotland, England, and I va- 

 luations, land. 



5. Trap-tuff, Jameson. 

 Tra P- tuff> Trap-tuff, Werner. 



Composi- Composition. This rock has a conglomerated struc- 

 tion. ture, and therefore has a basis or ground, with imbed- 



ded masses of various forms and sizes. 1 he ground 

 or basis varies in its nature ; sometimes jjt is wacke or 

 greenstone, or it inclines to basalt or amygdaloid. The 

 imbedded masses, which are of various roundish and 

 indeterminate angular forms, are of wacke, amygda- 

 loid, greenstone, and basalt ; and intermixed with these 

 are sometimes masses of sandstone, limestone, slate 

 clay, bituminous shale, and jasper. The masses vary 

 in size, from that of a pea, to several yards, or even 

 fathoms, in length, breadth, and thickness. 



Petrifactions. It sometimes contains truAks and Geognosy. 

 branches of trees, more or less bituminized. s ~"~v""" 



Structure. It occurs in beds, which are from a few * >etrifac- 

 inches to many fathoms in thickness ; is sometimes dis- s'^ucture 

 tinctly stratified, and occasionally it is disposed in glo- 

 bular distinct concretions, the balls varying from a few 

 inches to several feet in diameter, and also in columnar 

 concretions. 



Geognostic Situation. It occurs in imbedded masses, Geognostic 

 beds, and veins, along with the other rocks of the se- situation, 

 condary trap series already enumerated. 



Geographic Situation. A considerable portion of Ar- Geographic 

 thur Seat is composed of this rock; it abounds in many situation, 

 of our red sandstone and coal districts, and is very 

 frequently met with among the Hebrides, as in Canna 

 and Eigg. 



B. Secondary Porphyry, Jameson. 



Flcetz Porphir, German Mineralogists. 



The secondary porphyries are composed of felspar in Secondary, 

 various states of aggregation, hornstone, and pitchstone, porphyry, 

 with intermixed grains and crystals of felspar, quartz, 

 and other minerals. 



The following are. the rocks, arranged under this 

 head : 



1. Claystone. 2. Claystone porphyry. 3. Felspar. 

 4. Felspar porphyry. 5. Clinkstone. 6. Hornstoue 

 porphyry. 7. Pitchstone. 8. Pitclistone porphyry. 



1. Claystone, Jameson. 

 Thonstein, Werner. 



This rock, which is described in p. 336, appears to Claystone. 

 be felspar in a comparatively loose state of aggregation. 

 It is sometimes conglomerated, and then is named clay- 

 stone tuff. It is disposed in beds and veins in sand- 

 stone districts, and in coal-fields both in England and 

 Scotland. It is always an accompanier of rocks of the 

 secondary porphyry formation. 



2. Claystone Porphyry,. Jameson. 

 Thonstein Porphir, Werner. 



Claystone 

 porphyry. 



Composition. This porphyry, as its name intimates, Composi- 

 has a basis of claystone, in which imbedded crystals of tion. 

 felspar are contained. 



Structure. It occurs in columnar, tabellar, and glo- Structure, 

 bular distinct concretions; is sometimes conglomerated, 

 rarely vesicular, and occasionally imperfectly stratified. 



Imbedded Minerals. These are felspar, quartz, ei- imbedded 

 ther in grains or in double six-sided pyramids, mica, minerals, 

 augite, hornblende, and iron pyrites. 



Petrifactions. Trunks, branches, and twigs of trees, Petrifac- 

 occur imbedded in this rock, and these are generally 'ions, 

 penetrated or petrified with wood-stone. 



Geognostic Situation. it occurs in imbedded masses, Geognostic 

 beds and veins in secondary sandstone and limestone situation, 

 districts, and is sometimes associated with the secondary 

 trap-rocks. 



Geosrapliic Situation. The Pentlandand Ochil hills, Geographic 

 near Edinburgh, contain abundance of secondary por- situation, 

 phyry, and the same is the case in some districts in the 

 south and north of Scotland, and also in England and 

 Ireland It is a frequent rock in similar situations OH 

 the continent of liurope. 



3. Felspar, Jameson. 



This mineral occurs as a mountain rock, with red or Felspar, 

 grey colours, and either very minutely foliated, or in a 

 compact state. 



