Final answer:
The geologic law that discusses igneous inclusions is the principle of inclusions, stating that inclusions are older than the surrounding rock. Intrusive igneous rocks like granite are coarse-grained and may contain older inclusions, while extrusive igneous rocks cool quickly and are fine-grained.
Step-by-step explanation:
The geologic law that includes igneous inclusions is known as the principle of inclusions. This principle, developed by Charles Lyell, states that an inclusion, which is a piece of rock contained within another rock, must be older than the rock in which it is included. In the context of igneous rocks, if a mafic inclusion is found within a felsic rock such as granite, it indicates that the mafic rock had already solidified before the granite began to crystallize around it. Hence, the inclusion is indeed older than the surrounding rock. This relationship is significant because it can help geologists determine the order in which different geological events have occurred.
Intrusive igneous rocks are a key example related to this principle. These rocks form from magma that cools slowly deep within the Earth, resulting in a coarse-grained texture. For instance, granite is a common type of intrusive igneous rock composed mainly of feldspar and quartz, and may contain inclusions of other rock types.
On the contrary, extrusive igneous rocks cool rapidly at or near the Earth's surface, resulting in a fine-grained texture. This rapid cooling often prevents crystal growth, leading to a very different appearance compared to intrusive rocks.