CO885-(21-23) — Page 485

PUBLIC RECORD OFFICE

Reference :-

LITICO. 885

22 PUBLIC RECORD OFFICE, LONDON

ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO

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XI.

FOSSILS.

In collecting fossils it is obviously desirable that they should include as many different forms and be in as perfect condition as is possible. The exact bed in which each occurs should be ascertained and recorded, and if there are several fossiliferous bands the fossils from each should be clearly distinguished. Care should also be taken to distinguish between fossils belonging to the stratified rocks themselves and those found in fissures in them filled with later material, which has entered from above, but has in many cases consolidated to form hard rock. Such occurrences are frequent in limestones.

A good lens should be employed to look for minute fossils, which might escape notice on a cursory examination of the rock.

In slates fossils are rarely well seen on the cleavage face unless it happens to coincide with the bedding. Where this is not the case search should be made for the true bedding planes and efforts should be made to break the rock along these across the cleavage. It is on the fractures thus obtained that there is the best chance of finding fossils.

As a general rule fossils are difficult to detect in compact lime- stones except on a weathered surface such as is found in cliffs, old quarry faces and masonry.

Special attention ought to be paid to concretionary nodules which should be carefully split open to ascertain if they contain fossils. Sometimes the best results are obtained by first breaking them across the bedding and then splitting them along it.

Calcareous nodules in clays and shales may include shells and fish remains, while ironstone concretions in coal yield well pre- served plant fragments.

Limestones frequently contain siliceous concretions of which flints are the most familiar example. These afford not only organic remains which were originally siliceous hut calcareous and other organisms which have been converted into silica. When the interior is hollow it usually contains fine siliceous powder which consists of minute organisms such as sponge spicules. Where a limestone has been exposed to weathering and solution, remains of silicified organisms derived from it are frequently found in the cultivated fields above it, and in many desert areas siliceous concretions and silicified fossils scattered over the ground are the only evidence of thick beds of limestone which have been removed by the action of the wind. Calcareous organisms

which occur in soft easily weathered shale are in like manner found on the surface of arable land.

Phosphatic nodules, which are met with in limestones and other beds, also frequently contain fossils.

Special search should be made in the different deposits that overlie the rock floors of enves for vertebrate fossils and ancient human implements, utensils, remains of hearths and refuse heaps. Caves usually occur in limestone and when this is the case the older deposits are frequently covered by a layer of calcareous stalagmite. This should be removed and the material beneath

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examined. Too much stress cannot be laid on the necessity of exercising the greatest care in locating the exact position of every object. The horizontal position should be indicated on an accu- rate plan and the vertical position above a fixed datum expressed in figures. The plan should usually be supplemented by verticul

sections.

Limestone bills have frequently a deposit of secondary lime- stone or travertine at their base which may yield remains of con- siderable scientific importance, as well as being valuable as a source of pure limestone.

Old river gravel and loam and old lake beds left dry by the lowering of the level of the water also deserve careful search. If the ground is sufficiently soft, bones may sometimes be located by means of a long narrow rod of steel thrust downwards from the surface.

As a general rule no attempt should be made to clean fossils or remove them from the matrix except so far as may be abso- lutely necessary to reduce the specimen to a portable size. In this way fossils are more likely to be preserved uninjured, and the risk of the removal of a portion of the external surface or of inconspicuous appendages will be avoided. At the same time the nature of the matrix in which the fossil was found will be recorded in the most unequivocal manner.

Any separation of one portion of a fossil from another should be, if possible, avoided. Where a fossil consists of a number of distinct parts such as the palatal teeth of a shark or ray or a vertebrate skeletou, a slab showing them in the association in which they were found is of much greater scientific value than the same specimens separated from one another, and even in the case of distinct fossils, such as shells, a slab showing a number in association has a value of its own.

When a specimen, on account either of its size or of its condition, has to be divided into two Or more portions, whether these are separate bones of a skeleton or parts of the same object, it should be carefully sketched, and, if possible, photographed, before it is disturbed. At the same time each portion should be marked with a number to identify it, which should also appear on the sketch. It is also desirable that the nearest points of adjoining portions should bear an identical letter to show where they fit together and, as an additional precaution, this may be accompanied in each case by the number of the adjoining specimen in brackets. If a specimen is to be separated, a line of red or may not improbably come apart, along a crack, paint may be drawn across the crack at right angles to it before the specimen is disturbed, so that the two portions can be again brought together in their correct relative positions.

The importance of indicating the original orientation of rock specimen containing fossils has already been insisted on.

Sometimes hollows or "casts are found which represent the space formerly occupied by a fossil which has been removed. In such cases a portion of the rock sufficiently large to include the entire cavity should be taken, so that a plaster cast may ultimately be made of it.