98 



LITHOLOGICAL GEOLOGY. 



in each concretion) consisting of different minerals : for example, garnets 

 characterizing the centre, feldspar the middle layer, and mica the outer; and all 

 making a solid mass. The constitution of such concretions is very various. In 

 rocks containing feldspar they usually consist largely of feldspar, and some- 

 times, of feldspar alone, or of feldspar with some quartz. The concretions in 

 pitchstone and pearlstone (called spherulites) are almost purely feldspathic, and 

 often separate easily from the rock. 



105. Fig. 83 represents basaltic columns, like those of the Giants' Causeway, 

 haA'ing the tops concave: at each joint in the columns, in such a case, there 

 would be the same concavity, a convex and concave surface fitting neatly 

 together like a ball-and-socket joint. This tendency' to break with concave or 

 convex surfaces is another example of concretionary structure; and in the 

 example referred to, each column is an independent line of concretionary 

 solidification distinct from the others. This concretionary structure is often 

 wholly unobservable in the solid unaltered rock. But let it begin to decompose 

 by atmospheric agencies, and concentric or successive concave layers become 

 apparent; and sometimes they are so perfectly developed as to separate easily 

 and afford thin plates, or an imperfectly slaty structure. 



106. In some granite and sandstone, decomposition develops in like manner 

 a concretionary structure. The rock, after partial alteration, peels off in con- 

 centric layers, and a bluff of granite which has undergone the change some- 

 times appears as if made up of huge rounded boulders piled together, with earth 

 or crumbling rock between; in fact, each of the masses resembling boulders was 

 the centre of a concretion. A sandstone often looks like an excellent stone for 

 buildings, which, after an exposure of a few months, will fall entirely to pieces. 

 Complete immersion in water is often a protection to such stone; and they 

 may frequently be used architecturally for submarine purposes when not fit for 

 structures out of water. 



Fig. 84. 



Fig. 84 is a case of concretion in a sandstone alongside of a small fissure, 

 observed in Australia. The two concretions measured twenty feet across. 

 They consisted of layers from half an inch to two inches thick, which separated 

 rather easily. The rock elsewhere was without concretions. 



Fig. 85 is from an argillaceous sandstone which before consolidation had been 

 intersected by slender mud-cracks, and subsequently, on hardening, each areo- 

 let became a separate concretion. The action of the sea had worn the surface 

 and brought the structure out to view. 



