Here's a breakdown:

* Silica-deficient: Gabbro and basalt are indeed considered silica-deficient compared to other igneous rocks like granite or rhyolite. They typically have lower silica content (SiO2).

* High Magnesium and Iron: This is also generally true. Gabbro and basalt are mafic rocks, meaning they are rich in magnesium (Mg) and iron (Fe). These elements are key components of minerals like olivine and pyroxene, which are common in mafic rocks.

However, it's important to be precise with the terminology:

* Silica-deficient is a relative term. While gabbro and basalt have lower silica content than other igneous rocks, they still contain significant amounts of silica.

* High magnesium and iron is also relative. While they are richer in these elements than other igneous rocks, their composition can still vary depending on the specific geological context.

In summary: Gabbro and basalt are generally considered silica-deficient with high magnesium and iron content. This makes them mafic rocks, which are often dark-colored and denser than felsic rocks. However, it's crucial to remember that these are relative terms, and the exact composition can vary.