Subsurface Laboratory Methods 101 



possibilities, both as time-index fossils and as indicators of environment 

 of deposition. 



Calcareous algae often coat and fill voids of other fossils. Certain 

 algae occur symbiotic with other organisms and produce growths that 

 make characteristic and often rather common fossils (e.g., Archimedes). 



Fossil algae are among the oldest fossils known; in fact, they are 

 the only group that has been found in considerable numbers in pre- 

 Cambrian rocks. 



Diatoms 



Diatoms are microscopic unicellular plants belonging to the phylum 

 Thallophyta. The skeleton or frustule, composed of opaline silica, com- 

 monly consists of two shallow, disk-shaped halves (example: Craspedo- 

 discus) , one of which fits into the other similar to a flat pillbox. Other 

 forms are elongate and bilaterally symmetrical (example: Glyphodiscus) 

 with respect to an axial strip. Individual specimens may range in diame- 

 ter from 0.1 to 0.15 mm. Magnifications up to X 200 are generally re- 

 quired to study this group of microflora. It has been estimated that a 

 cubic inch of some diatomites (deposits composed essentially of diatoms) 

 contains as many as two billion frustules. 



Classification of fossil diatoms is based on the size, shape, and sur- 

 face ornamentation of the frustule. The intricacy and complexity of sur- 

 face ornamentation of certain species are fantastic and remain surprisingly 

 uniform. (See fig. 44). 



Although diatoms live under a wide range of environmental con- 

 ditions, they are highly selective as regards sunlight, temperature, salinity, 

 turbidity, and percentage of silica in the water. Some species live only 

 in fresh water, whereas others thrive only in saline water. Some forms 

 attach themselves to objects, although most are of the planktonic variety. 



The fats secreted by diatoms are considered by some petroleum 

 geologists to be the source of much of the oil and gas being produced 

 from the Miocene sediments of California. 



Diatomite has a world-wide distribution, although the size of the 

 individual deposits is not large. Marine Miocene diatomaceous deposits 

 occur in California, Maryland, Virginia, Algeria, and Denmark. Large 

 fresh-water deposits are found in North America, Europe, and Japan. 

 The largest deposit in the world is that at Lompoc, California. This de- 

 posit consists of 1,400 or more feet of stratified diatomite in the Monterey 

 series of the upper Miocene and covers an area of 4,000 acres. Estimated 

 reserves have been placed at 100,000,000 tons. The more important com- 

 mercial uses of diatomite are as mineral filters, paint filler, insulation, 

 building blocks, and abrasives. 



Diatoms have had a relatively short geologic history ranging from 

 the Late Cretaceous to the Recent. They were common in the Cretaceous, 

 became prolific in the Tertiary, and reached their highest development 



