One can scarcely doubt that this gabbro is the plutonic equivalent of the prevailing grey pyroxene-andesites.
Since plutonic rocks are of very rare occurrence in this island, the Nawi gabbro has a particular interest.
It is very probable that this gabbro forms the axis of the range; and we have here no doubt one of the oldest of the mountain-ridges in the island.
When, however, we look at the exposed gabbro at the other end of the ridge, we have to carry the period much farther back, since here the superjacent surface volcanic rocks have been stripped off completely.
This ridge, which is about 700 feet in height and forms the termination of the Valanga Range, is composed of altered grey hornblende-pyroxene-andesites and of similar holo-crystalline rocks representing the gabbro or plutonic type of the same.
On the head-waters of the river is situated the hamlet of Nawi, where, as mentioned on page 211, a plutonic rock of the gabbro type occurs.
Segregations of iron ores, such as ilmenite, usually with pyroxene or olivine, occur in association with some gabbro and anorthosite masses.
Very often a mass of normal gabbro with typical igneous character passes at its margins or along localized zones into foliated rocks of this kind, and every transition can be found between the different types.
Rocks of the gabbro group, though perhaps not so common nor occurring in so great masses as granites, are exceedingly widespread.
It is traversed in places by dikes and sills of diabase or dolerite, while bosses of more or less altered gabbro rise through it.
These are caused either by old intrusions of diabase and gabbrowhich have undergone modifications, or by later ones of dolerite.
These volcanic materials are pierced by serpentine, gabbro and granite.
Gabbro appears on the shore north of Lendalfoot, while on the Byne and Grey Hills south of Girvan there are patches of granite and quartz-diorite which seem to pass into more basic varieties.
The granites pass into gneiss and granulite; the gabbros into flaser gabbro and amphibolite; the slates often contain andalusite or chiastolite, and show transitions to mica schists.
The rocks of this group accompany gabbro and serpentine, but the exact conditions under which they are formed and the significance of their structures is not very clearly understood.
Do you know anything of the effects of polarized light, the sight of a slice of olivine-gabbro for instance between crossed Nicols?
Five yards away, perhaps, a great mass of purple gabbro hung over a patch of nearly snowless moss.
The serpentine and gabbro formation is characteristically of an eruptive nature.
The rocks in Strawberry Mountain and to the east are mostly lavas which poured out over the land, whereas the Canyon Mountain part of the range consists of gabbro and peridotite which were intruded at great depth, like granite.
It may stop far below the surface and cool slowly, forming coarsely crystallized rocks of the granite and gabbro types.
The magnetite crystals of these deposits interpenetrate with the other constituents of an igneous rock, commonly of a gabbro type, and the deposits themselves are essentially igneous rocks.
The igneous rocks are commonly acid intrusives of a granite or porphyry type, less commonly intrusives of gabbro and diabase and surface lavas of rhyolite and basalt.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "gabbro" in a variety of sentences. We hope that you will now be able to make sentences using this word.
