Yes, that's correct! Gabbro and basalt are indeed considered silica-deficient with high magnesium and iron content. Here's why:

* Silica Content: Both gabbro and basalt are classified as mafic rocks, meaning they are relatively low in silica (SiO2) content. This is in contrast to felsic rocks like granite, which are rich in silica.

* Magnesium and Iron: Mafic rocks like gabbro and basalt are rich in magnesium (Mg) and iron (Fe). These elements contribute to their dark color and relatively high density.

Here's a simplified way to remember it:

* Felsic rocks: High silica, low Mg & Fe (light-colored, less dense)

* Mafic rocks: Low silica, high Mg & Fe (dark-colored, denser)

Further information:

* Gabbro is an intrusive igneous rock, meaning it cools and solidifies slowly beneath the Earth's surface.

* Basalt is an extrusive igneous rock, meaning it cools and solidifies quickly on the Earth's surface, often from volcanic eruptions.

Let me know if you have any more questions about rocks!