CONTENTS.
DIRECTIONS FOR THE COLLECTION OF GEOLOGICAL SPECIMENS.
PUBLIC RECORD OFFICE
Reference :-
C.O. 885
22 PUBLIC RECORD OFFICE, LONDON
I.
INTRODUCTORY.
When a systematic geological survey of a district has not yet been carried out, useful work may be done by collecting speci- mens and forwarding them to a compelent geologist for examina- tion. The specimens must however be carefully selected and properly cared for, and each must be accompanied by a clear statement not only of the exact locality in which it was found but also of its mode of occurrence.
If a geological survey is afterwards undertaken, the specimens and information which have been obtained will prove of con siderable value in enabling the members of the survey to under- stand from the first the nature of the task before them, and make their preparations accordingly.
Nor does the collection of specimens by amateur workers cense to be of value when the official surveyors have commenced, or even completed, the geological mapping of the country; for new opportunities of ser uring specimens, as well as of obtaining in- formation of geological importance will constantly be furnished by the opening up of quarries, the sinking of wells and bore holes, and the execution of public works such as roads and railways. It must however be clearly understood that the mere collection of specimens can never be a substitute for a systematic survey by a trained geological staff.
In the following notes I have assumed no geological acquire- ments on the part of the collector, but obviously any knowledge that he may possess of the subject will render his work of greater value. There is indeed no study in which there are so many opportunities for local workers or such need of their assistance as in geology. Everywhere there are special problems to he solveil and any spot may furnish information which will throw much needed light on the geology of a whole district.
No attempt has, however, been made in these pages to provide further instruction in geological principles than appears to be absolutely necessary to enable the collector to furnish some in- formation as to the relation of the specimens collected to the rocks of the locality where they were obtained, and the main purpose of this publication is to give such directions as will secure that the specimens are properly selected, that their source is suffi- ciently identified and that they are packed so as to suffer as little injury as possible in transport.
The author has relied mainly on his own experience of geological work in tropical countries, but he has incorporated many of the directions for the collection and packing of fossils and minerals to be found in the "Instructions for Collectors issued by the
(97759*—2.) „Wt. 34452—O 24, 600, 9/13, D & 8,
f.-Introductory
PAGE.
3
II.-Rocks, Minerals and Fossila
III.---Types of Rocks
IV.-Inclined, folded, and fractured Rocks
5
V.-Economic Products
G
VI. Decomposition and Weathering
G
VII. What to collect...
7
VIII. Breaking off specimens
8
IX.-Wrapping up and Packing
9
X.-Minerals and Meteorites
11
XI.-Fossils
12
***
XII-Clay and Shale...
15
XIII-Sand and Gravel
XIV.-Oil, Water, and Gas
XV.—Entries in the Note Book
***
16
17
18
246
ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TOPage 481
PUBLIC RECORD OFFICE
Reference :-
TTTTT CO. 885
22 PUBLIC RECORD OFFICE, LONDON
ALLY WITHOUT PERMISSION OF THE COPYRIGHT PHOTOGRAPH-NOT TO BE REPRODUCED PHOTOGRAPHIC-
British Museum (Natural History), 3rd Edition, 1906, and is also indebted for some suggestions to the section on Geology in "Hints to Travellers" published by the Royal Geographical Society, 9th Edition, 1906, Vol. II., originally compiled by the late W. T. Blanford and revised by Professor E. J. Garwood, and to the "Lehrbuch der praktischen Geologie" by Professor Keilhack, 2nd Edition, Stuttgart. 1908. He must also acknowledge the information and advice he has received from Dr. C. W. Andrews of the British Museum (Natural History) as to the preservation of vertebrate skeletons and from Mr. T. Crook and Mr. S. J. John- stone of the Imperial Institute on the collection of economic samples.
II.
ROCKS, MINERALS, AND FOSSILS.
The specimens which should form part of a representative gen. logical collection of a district are usually classed under the head- ings of rocks, minerals, and fossils, though there are many that might equally well be placed in more than one of these categories. The term "rock" is not confined by the geologist to hard masses such as those to which it is applied by general usage, but may include any material which forms a considerable portion of the earth's crust, whether it be hard and tough like granite, soft like clay, or as incoherent as sand.
The word "mineral" on the other hand is restricted by the geologist to a substance of definite chemical composition and characteristic physical properties, which is a constituent of rock masses or is found in fissures or other cavities within them. Most rocks are in fact made up of grains or crystals of different minerals.
By miners, however, the term "mineral" is used in the more general sense of any thing of economic value obtained by mining operations from the earth's crust. Thus coal would be con- sidered as a mineral from their point of view, though scientifically it has no claim to the title; but products employed for building or road making, such as granite, sandstone, limestone, gravel, sand, and clay are not as a rule referred to as minerals even by the miner, probably because they are usually quarried and not mined.
Under fossils are comprised all remains of plants or animals including, for instance, stems, leaves, shells and bones, as well as mere indications, such as footprints, of the former existence of a living organism. Fossils are frequently of the greatest value in determining the relations of the rocks in which they occur andl the geological structure of the country.
III.
TYPES OF ROCKS.
There are some rocks which usually occur in layers, described by geologists as **beds or *strata," that were originally practically horizontal but have in many cases been subsequently
5
tilted and thrown into undulations, They are often referred to as **sedimentary stratu," us they have as a rule been laid down as sediment by water. Lavas and volcanic ashes are also found in parallel flows and beds, which may either occur alone or inter- stratified with sedimentary rocks.
Another class of rocks includes those which have solidified underground from molten material of the nature of lava, that has forced its way into the rocks of the earth's crust. These intrusive rocks,' as they are called, sometimes occur as sheets or "sills
between sedimentary strata, sometimes as dykes," wall-like masses, usually approximately vertical, traversing other rocks, and sometimes form great coarsely crystalline masses, which usually extend over a considerable area at the surface. Of these massive intrusions, frequently referred to us plutonic rocks, granite is not only the best known but by far the commonest and most extensive representative.
There is another great group of rocks, the members of which resemble the massive intrusive rocks in their crystalline character and the sedimentary rocks in showing a kind of bedding or Stratification, described as their “foliation." They occur over extensive tracts in our African Colonies and Protectorates. Those which except for their foliation resemble rocks of the granite type are called * gneisses and those in which the appearance of bedding is well marked, "crystalline schists.”
*
IV.
INCLINED, FOLDED, AND FRACTURED ROCKS. Where a bed or layer of any kind is inclined, the angle that it makes with the horizontal plane is termed the **angle of dip "; while the direction of dip "is the compass direction towards which the stratum slopes downward. Thus a bed may dip at an angle of 5o to the north, or north-north-east, or north 40° east, as the case may be.
The line in which the surface of a stratum meets a horizontal plane is obviously at right angles to the direction of the dip and is termed the "strike.' It is usually expressed in terms of both its directions; thus, if the dip is to the north-east, the strike is north-west and south-east. It is unnecessary to give the strike as well as the dip, unless the latter is uncertain or indeterminable, us it is, for instance, if the rocks are vertical.
Some strata, especially sandstones, have a local "cross bed- ding ("current" or “false" bedding of some authors) making an angle with the general bedding of the rocks of which they form part. When the dip is spoken of without qualification, it is the dip of the rocks as a whole that is meant not the dip of the cross bedding.
Other beds, usually those originally of a clayey or shaly nature, which have been much compressed by lateral pressure, acquire a fissility, known as slaty cleavage, at right angles to the pressure. This looks at first sight like true bedding but the latter can usually be recognized as it shows a succession of beds of different composition and characters. The angle of the true dip differs