156 -- @EOGNOSY. [Boos I 
sand is exposed on a coast-line the shell-fragments are in great 
measure ground into calcareous mud and removed. 
Mr. Sorby has recently shown that by microscopic investigation 
much information may be obtained regarding the history and source 
of sedimentary materials. He has studied the minute structure of 
modern sand, and finds that sand-grains present the following five 
- distinct types, which, however, graduate into each other. 
1. Normal, angular, fresh-formed sand, such as has been derived 
almost directly from the breaking up of granitic or schistose rocks. 
2. Well-worn sand in rounded grains, the original angles being 
completely lost, and the surfaces looking like fine ground glass. 
3. Sand mechanically broken into sharp angular chips, showing 
a glassy fracture. 
4, Sand having the grains chemically corroded, so as to produce 
a peculiar texture of the surface, differing from that of worn grains 
or crystals. 
5. Sand in which the grains have a perfectly crystalline outline, 
in some cases undoubtedly due to the deposition of quartz upon 
rounded or angular nuclei of ordinary non-crystalline sand.* 
The same acute observer points out that, as in the familiar case of 
conglomerate pebbles, which have sometimes been used over again 
in conglomerates of very different ages, so with the much more 
minute grains of sand, we must distinguish between the age of the 
grains and the age of the deposit formed of them. An ancient 
sandstone may consist of grains that had hardly been worn before 
they were finally brought to rest, while the sand of a modern beach 
may have been ground down by the waves of many successive 
geological periods. 
Sand taken by Mr. Sorby from the old gravel terraces of the River 
Tay was found to be almost wholly angular, indicating how little 
wear and tear there may be among particles of quartz +, of aninch 
in diameter, even though exposed to the drifting action of a rapid 
river.” Sand from the boulder clay at Scarborough was like- 
wise ascertained to be almost entirely fresh and angular. On the 
other hand, in geological formations, which can be traced in a given 
direction for several hundred miles, a progressively large pro- 
sate of rounded particles may be detected im the sandy beds, as 
— has found m following the greensand from Devonshire 
to Kent. 
The following names are applied to forms of sandy or gravelly 
detritus when consolidated. 
Conglomerate (Puddingstone)—A name given to any rock 
formed of consolidated gravel or shingle. The component pebbles 
are rounded and waterworn, They may consist of any kind of rock, 
though usually of some hard and durable sort, such as quartz or 
quartz-rock. A special name may be given according to the nature 
of the pebbles, as quartz-conglomerate, limestone-conglomerate, 
" Address, Q. J. Geol. Soc. xxxvi. 1880, p.58, * See Book ITI, Part IL, Sections ii. § iii 


