Silica is also known as Diatomaceous Earth . Made up of 84% Silicon Dioxide (Silica).* The typical chemical composition is 84% silica, 5% sodium, 3% magnesium and 2% iron and it contains other essential trace minerals including calcium, titanium, boron, manganese, copper and zirconium.

• Silica is not dirt! It is a natural grade diatomite
• Silica is totally organic and safe.
• Pure Fresh Water Food Grade - it is taken from fresh water deposits and is the purest form available
• Silica is mother nature's product with no harm to the environment, pets or to people

Some benefits of feeding  Silica :

• A source of 14 essential trace minerals
• All natural parasite control & natural anthelmintic
• Supports the repair process and is essential for tendons, ligaments and bones
• Supports natural stem cell production
• Promotes & supports a healthy bowel and respiratory function
• Balances stomach PH  and helps dogs stop eating their stools
• Naturally controls internal and external parasites by mechanical action
• Promotes & supports healthy skin, hair,  and hooves and nails
• No resistance development possible
• Completely Safe and Non-toxic
• Used for centuries in human and animal health

We have two types of diatomaceous earth available:

How does calcium Bentonite clay work?
Bentonite clay carries a uniquely strong negative ionic charge which causes it to `magnetically` attract any substance with a positive ionic charge (i.e., bacteria, toxins, metals, etc.). These substances are both adsorbed (sticking to the outside like Velcro) and absorbed (drawn inside) by the clay molecules. Your body doesn`t digest clay, so the clay passes through your system, collecting the toxins, and removing them as the clay is eliminated. It`s like a little internal vacuum cleaner. In his book The Clay Cure, Ran Knishinsky states it this way:
"The clay`s immediate action upon the body is directly on the digestive channel. This involves the clay actually binding with the toxic substances and removing them from the body with the stool. It performs this job with every kind of toxin, including those from the environment, such as heavy metals, and those that occur naturally as by-products of the body`s own health processes, such as metabolic toxins. The clay and the adsorbed toxins are both eliminated together; this keeps the toxins from being reabsorbed into the bloodstream."
So while it currently may be impossible to avoid these contaminants, using calcium Bentonite clay can remove them from our system.** read more...
_{* Perma-Guard (Pure Food Grade ) has been reviewed and is now listed with the Organic Materials Review Institute.** These statements have not been evaluated by the FDA.}

Diatomaceous earth is a marine sediment consisting primarily of the skeletons of diatoms, which are microscopic algae. The skeletons consist almost entirely of
silica, though other material may be present in the sediment. It does not swell when wetted.

Bentonite Clay is a montmorillinite clay mineral that results from the weathering of volcanic ash deposits and that swells when it gets wet. It has an entirely geological origin, whereas diatomaceous earth has an organic origin. Both, however, contain silica as well as traces of other minerals.

USES: Diatomaceous Earth is sometimes brushed through the fur or feathers of animals or put in their bedding to kill insect parasites. It is also often added to horse feed to keep the food from clumping and as a source of trace minerals.

"The governments of the United States and Canada recognize that fossil shell flour is safe to use in animal foods in an amount not to exceed 2% by weight of the total feed ration. When added at this percentage, fossil shell flour prevents “clumping” of feed particles by keeping them separate, so there
is improved flowability, mixability and handling of the animal feed. This in turn, creates two big advantages to the animals who consume fossil shell flour in their recommended feed ration. First, acting as an anti-caking agent to prevent “clumping” of feed particles, the surface area of feed exposed to the digestive processes - both bacterial and enzymatic - is increased and therefore more feed is actually digested and utilized. Secondly, fossil shell flour contains a small amount of 14 trace minerals."

Diatomaceous Earth is used by holistic practioners for detoxifying and cleansing the body. It is also supposed to kill internal parasites by abrading their bodies.

"Many harmful things entering the body have a positive charge. Silica is a semi-conductive mineral which when warmed by body heat becomes negatively charged and gives off electrons. These negatively charged mineral ions and/or individual shells attract bad microbes, free radicals, positively charged waste and other harmful things. Acting as magnets, the negatively charged shells and/or ions attract and absorb positive things that are small enough to go through the holes. Add a sugar molecule and you can trap toxins into the porous food-grade DE particle which is then excreted safely out of the body. Because of the strong charge, each shell can absorb a large number of positively charged substances, whether they be chemical or in the form of bacteria or viruses. They pass on through the stomach and intestine, taking these harmful substances out of the body."
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Bentonite is also used in holistic medicine to detoxify the body. Holistic practitioners believe it absorbs toxins such as mercury and that it goes where the harm is and selectively absorbs bacteria, pus and so on.

"“When clay is consumed, its vital force is released into the physical body and mingles with the vital energy of the body, creating a stronger, more powerful energy in the host. The natural magnetic action transmits a remarkable power to the organism and helps to rebuild vital potential through the liberation of latent energy. When the immune system does not function at its best, the clay stimulates the body’s inner resources to awaken the stagnant energy. It supplies the body with the available magnetism to run well. Clay is said to propel the immune system to find a new healthy balance and strengthens the body to a point of higher resistance.”

Bentonite also has been used as a poultice on wounds.

"Applied in poultices and compresses, bentonite clay provides numerous benefits. It can speed up the healing process for sores and ulcers and aid in the rebuilding of healthy tissues and cells, and even of fractured bones and vertebrae."

(Bentonite is used in making clumping clay cat litter because of its swelling tendency.)

If you really want to get into the holistic nitty gritty go to http://www.safesolutionsinc.com/40_-_Detoxification.pdf and read the whole thing. It's 50 pages long or I would cut and paste it here. 
Sources: http://www.potters.org/subject02599.htm  http://www.shirleys-wellness-cafe.com/birds.htm petcaretips.net/clay-clumping-cat-litter.html