Option 1: Basalt is an igneous rock that is formed from solidified lava or magma. It is not typically found in sedimentary rocks, so this option is not the correct answer.

Option 2: Silica refers to the compound silicon dioxide (SiO2), which is a common mineral found in many types of rocks. While silica can be found in sedimentary rocks, it is not exclusively associated with them. Therefore, this option is not the correct answer.

Option 3: Shale is a type of sedimentary rock that is formed from the accumulation of fine-grained sediments, such as clay and silt, over time. It is commonly found in sedimentary rock layers and is known for its ability to split into thin layers. This option is the correct answer because shale is indeed generally found in sedimentary rocks.

Option 4: Magnesium is an element and not a rock type. While magnesium can be found in certain minerals within sedimentary rocks, it does not typically characterize sedimentary rock formations as a whole. Therefore, this option is not the correct answer.

In summary, option 3 - shale - is generally found in sedimentary rocks, making it the correct answer.