Subsurface Laboratory Methods 129 



the sandy and silty groups on the basis of mineralogic composition is 

 desirable in light of the recent trend that attempts to tie up tectonics and 

 sedimentation. Figure 54, modified from Pettijohn,^^ is a mineralogic 

 classification of sands and silts that merits consideration by subsurface 

 workers. Clastic limestones, that is, those in which the grains have been 

 transported to their place of deposition, should be distinguished from 

 limestones in which no clastic texture can be recognized. 



To the main rock name, modifiers are added to permit full description 

 of the sample. As suggested by Krynine,^^ it is possible to apply to sedi- 

 ments the basic standardized descriptive sequence that has been used for 

 many years for igneous rocks, namely, color, subtexture, varietal minerals 

 and cement, and finally the main rock name. If necessary, terms describ- 

 ing structure may follow color. Thus one could have a "gray, cross- 

 bedded, fine-grained, glauconitic, dolomitic, quartz sandstone." For ease 

 in plotting sample data and in picking out changes in lithology, it is con- 

 venient to capitalize the main rock name and place it first, and to abbrevi- 

 ate as much as possible, as "QTZ SS, gy, x-b, f-gr., glauc, dol.," for the 

 example above. In commercial work it is not possible to give as complete 

 descriptions as might be desirable in purely scientific research, but the 

 description should be made as complete as possible in the time available. 



Porosity and Permeability 



Porosity is the percentage of total volume of a rock not occupied 

 by mineral components. Pores may differ in size and may be connected 

 or isolated. In magascopic or binocular examination, porosity usually 

 can be seen best in clean, dry samples. 



Permeability is the fluid-transmitting capacity of a porous material. 

 Permeability is not necessarily a function of porosity. Clays, for example, 

 may be very porous but relatively impermeable. In megascopic or binocu- 

 lar examination a rough measure of permeability may be made by ob- 

 serving how rapidly a drop of water will soak into a dry fragment. The 

 presence of individual sand grains or oolites, rather than clusters of such 

 grains, suggests slight cementation and consequently high porosity and 

 permeability. 



Subsurface samples may be examined wet or dry. Both methods 

 have advantages and disadvantages and when time permits a combination 

 of both wet and dry study is advisable. Study of dry samples is faster 

 and permits better observation of gross texture, porosity, permeability, 

 sparkling, and some color differentiations. Study of wet samples is ad- 

 vantageous when the samples are not clean. Some textures and colors 

 also can be seen better in wet samples than in dry samples. Calcareous 

 samples that have been etched with dilute hydrochloric acid are usually 

 studied when wet. It is important to indicate whether color has been 



^ Fettijohn. F. J., Sedimentary Rocks, p. 526, New York, Harper & Brothers, 1949. 

 ^ Krynine, P. D., op. cit. 



