1891. | DERBY—ON ACCESSORIES OF ROCKS. 201 
have to be left out of account, or be identified by a mere guess. Rocks 
too. much decomposed for section making will not only give the 
accessories in a perfect state of preservation, but also some grains of 
the essential elements that have escaped decay. If sufficiently decom- 
posed to be reduced to a pulp by the pressure of the hand under water, 
the washing may be done at the nearest stream or pool, and residues 
representing the really essential part of many pounds+ weight of rock 
can be carried home in the pocketbook. ‘The batéa can be as readily 
carried on excursions as a hammer, and in regions of decomposed 
rocks will often prove quite as useful. For wrapping the residues, the 
little books of cut paper for cigarette smokers have been found very 
convenient. If the rock is still too hard for washing in the field, 
specimens that otherwise would not be worth carrying home will repay 
transportation for examination in the laboratory. 
A process of what may be called reconstructive geology is thus 
rendered possible. Several applications of such a process have already 
been made with complete success, the clue afforded by the heavy 
residue of a rock completely reduced to clay having been followed 
through successive stages of decay to the sound rock. For purposes of 
identification only material decomposed zm” st¢u should be washed ; but 
if a quantity is desired, the accumulations of sand in rain-runs or 
stream beds may be washed with advantage, as they are natural concen- 
trates of the heavy minerals with the quartz of the adjoining rocks. 
The minerals most commonly met with in decomposed rocks and 
in these natural concentrates are the iron and titanium minerals : 
(magnetite, pyrite, ilmenite, rutile, anatase, sphene, perofskite,) zircon, 
monazite, xenotime, apatite, tourmaline, garnet, staurolite, epidote, 
orthite, corundum, spinel, cassiterite, etc. The general distribution of 
these minerals in sound rocks is pretty well known, though the batéa 
will often reveal their presence where they would otherwise be unsus- 
pected. ‘The following observations on their occurrence in the residues 
of decomposed rocks may be of service to those who may feel tempted 
to try this method of investigation. The greater part of the minerals 
of the above list appear unchanged even in the most completely decom- 
posed material; magnetite is occasionally altered to martite and 
limonite, pyrite to limonite, and ilmenite to an aggregate of microscopic 
needles of rutile; rutile (sagenite) and anatase, not present in the 
sound rock, may appear in the decomposed form, being formed appar- 
ently at the expense of ilmenite ; epidote may appear in the partially 
decomposed rock, but be lacking in the sound and totally decomposed 
forms, while orthite, sphene and perofskite seem at times to disappear 
