HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 537 



Conglomerate or puddingstone is merely a coarse sandstone ; it differs 

 from sandstone only as gravel differs from sand. (See specimens 25647 

 and 38199, which are but loosely consolidated gravels ; also 38512, from 

 Nantasket, Massachusetts.) The beautiful sample No. 70645 from Dev- 

 onshire (?), England, is composed of rounded pebbles of jasper cemented 

 by siliceous matter, lu No. 28744, from near Point of Rocks, Maryland, 

 is shown a Triassic conglomerate composed of both calcareous and 

 siliceous pebbles, some of which are angular and some rounded, the 

 rock thus presenting a form intermediate between conglomerates and 

 the breccias. Sample No. 72795, from the Siskiyou Mountains, is per- 

 haps rather a pebbly sandstone than a true conglomerate, being com- 

 posed of large rounded pebbles in a finer grained or sandstone matrix. 



Specimens 73080 and 73081, from Gallatin County, Montana, will illus- 

 trate the frag mental nature and origin of these rocks. Such were 

 formed near the shore line of a now extinct lake, and show the irregu- 

 lar admixture of fine sand and rounded pebbles of quartz, feldspar, 

 and other minerals, such as may not infrequently be seen on the mar- 

 gins of lakes and rivers of the present day. 



Greywacke or Grauwacke is an old German name for brecciated 

 fragmental rocks made up of argillaceous particles (Specimen No. 

 38156). The name is now little used. Other names, as flagstone, brown- 

 stone, and freestone, are applied to such of these rocks as are used 

 for economic purposes, but which ueed not be referred to here. Shale 

 is a somewhat loosely defined term, indicating structural rather than 

 chemical or mineralogical peculiarities. The word is perhaps best used 

 in its adjective sense, as a shaley sandstone or limestone. By many 

 authors it is used with reference more particularly to thinly stratified 

 or laminated clayey rocks (Specimen No. 36010). Itacoluinite or flexi- 

 ble sandstone is a feldspathic quartzite from which the interstitial feld- 

 spathic portions have been removed by decomposition leaving the in- 

 terlocking quartz grains with a small amount of play between them. 

 The rock is iu no sense elastic but merely loose jointed (Specimen No. 

 11951). (See also larger samples in special exhibit.) 



Breccia is a fragmeutal rock differing from conglomerate in that the 

 individual particles, having suffered but little attrition, are sharply an- 

 gular instead of rounded. Specimen No. 37924, from the Yellowstone 

 National Park, is a good type of these rocks. (See also Fig. 1, Plate 

 cxix.) No. 72794 is a chert breccia, the cementing material of which 

 is sphalerite. 



2. The argillaceous group — Pelites. — The rocks of this group are com- 

 posed essentially of a hydrous silicate of alumina, which is the basis of 

 common clay. In nature they are almost universally more or less im- 

 pure through the presence of siliceous sand, calcareous or carbonaceous 

 matter. They have originated in situ from the decomposition of felds- 

 pathic rocks or as deposits of fine mud or silt on the bottom of an 

 ocean, or more rarely a lake or river. The older formations of argilla- 



