Subsurface Laboratory Methods 117 



conditions of transportation and deposition, and post-depositional changes 

 may be expected to increase in importance. Sedimentary rocks have many 

 properties from which their past history may be deduced with varying 

 degrees of success. Most rocks are made up of a large number of grains 

 and these grains have the individual properties of size, shape, roundness, 

 surface texture, orientation in space, and a variety of other properties 

 that are dependent on and can be grouped under composition. In the 

 aggregate the particles have mass properties. These include average size 

 and a spread about that average, average shape, roundness, and orienta- 

 tion, and a spread about these averages. Porosity, permeability, color, 

 mud cracks, bedding, ripple marks, and the kind and degree of induration 

 are some of the many other mass properties. 



Of these properties, some, like composition, may be inherited from 

 the original source material; some, like shape and roundness, were devel- 

 oped as the particles were transported from the source to their place of 

 deposition; some, such as orientation, size, and bedding, reflect the envi- 

 ronment of deposition; and some, like porosity and permeability, at least 

 in part reflect post-depositional changes. It is with the study and inter- 

 pretation of these individual and mass properties that this section is pri- 

 marily concerned. 



Choice of Methods 



Many methods are used to study the mineralogy of subsurface sam- 

 ples. Some properties of the samples can be determined by megascopic 

 examination, supplemented by simple chemical and physical tests. Other 

 properties can be studied best with a binocular or petrographic micro- 

 scope. Certain minor constituents may be concentrated as insoluble or 

 heavy-mineral separates before binocular or petrographic examination. 



In selecting methods of study careful consideration must be given 

 to the objectives of the investigation and to the type of data required to 

 achieve these objectives. Methods that provide satisfactory information 

 for one problem or one area may be entirely unsuited to another problem 

 or another area. The hazards of unthinking application of techniques and 

 methods cannot be overemphasized. 



Naturally the type, number, size, and location of samples that are 

 available are controlling factors in outlining a program of investigation 

 and in choosing methods of analysis. Rotary cuttings, cable-tool cuttings, 

 and cores may yield diff"erent types of data. Given certain samples, how- 

 ever, choice of methods depends on the answers to two questions, namely, 

 what methods will provide the data needed to solve the problem under 

 consideration, and which method will yield the information with the 

 minimum expenditure of time and money. 



As an example, many correlation problems may be solved by rapid 

 megascopic or binocular examination of a few samples or a series of 

 samples. Where this is possible, further extensive laboratory examination 



