"top soil" is very generic and broad in its description and its contents. top soil is often scraped from the land when a housing or business development is being built - thus the name "top" soil. It has no standards and can contain all sorts of things depending where in the country you live.
As Carlton said the bagged soils often contain fertilizers and can also contain systemic biocides (pesticides, herbicides).
A "soilless" mix which contains no perlite is your best bet. Check with a local nursery (not Garden Center) even if they are wholesale and speak with them about possibly buying thru them.
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Chameleon Help & Resource Info
1.0 Nosy Be Panther Chameleon - Cyrus
0.1 Veiled Chameleon - Luna. She's now hanging from her big jungle gym in the sky
1.0 Beardie - Darwin
1.1.1 Side-blotched lizards - Ana and Stan
0.2 felines - Kyndra and Lita
0.1 African Clawed Frog - Skippy
0.1 Chilean Rose Hair Tarantula - Rosa Leigh, Died 4/21/06 
0.1 Goliath Bird-Eater Tarantula - Natasha, donated to science 4/4/06
?.? Pinktoe Tarantula - no name yet