1. geologic time
2. stratum
You could have just googled these...
There are multiple reasons for the variations in type, quality, and quantity in minerals, such as:
- place of formation, on the surface, or inside the ground and how deep inside.
- pressure; extremely important for the structure of the minerals as well as the mixing of mineral types.
- temperature; similar to the pressure, in fact in combination with it, affects the structure of the minerals.
- tectonic activity and volcanic activity; cause the other factors and are also responsible for the quantity of minerals.
One important factor is location - the reason being that the United states are a very big country and that you cannot surely measure the weather based on one location and from that deduce how the weather will change for all locations in the US.
Another important factor is the models used in the analysis - one has to realize and know that such data sollections is limited by the analysis in terms of its accuracy, a certain forecast doesn't necessarily mean that it will also be absolutely correct.
