These two rocks hold essentially the same relation as regards origin and structure to the basic lavas that petrosilex and felsite do to the acidic lavas.
Petrosilex and felsite are more generally porphyritic than any other rocks; and they are commonly called porphyry.
There is no limit to the proportion of the quartz and feldspar which may crystallize out in this way, and thus we find a perfectly gradual passage from normal petrosilex or felsite to thoroughly crystalline granite and syenite.
The typical felsite or petrosilex is composed entirely of felsitic matter, and is perfectly homogeneous, like flint or jasper, which it closely resembles in hardness and other physical characteristics.
We find here nothing new in composition; but petrosilex and felsite are simply the crystalline rocks which we have already studied, repeated under a different texture.
These rocks approach towards true granites in one direction, and through quartz-porphyry and felsite to rhyolite in another--probably depending upon the conditions of cooling and consolidation.
It is worthy of note that the dykes and sheets of felsite are seldom pierced by the basalt dykes and are probably about the most recent of the intrusive rocks.
The felsite sheets are well represented in Holy Island.
But felsiteor microfelsite is still the generally accepted designation for that very fine-grained, almost crypto-crystalline substance which forms the ground-mass of so many rhyolites, dacites and porphyries.
Some rocks are felsitic in parts but elsewhere glassy; and it is not always clear whether the felsite is an original substance or has arisen by the devitrification of primary glass.
The designs were rudely pecked on the moderately smooth felsite cliffs on a nearly perpendicular wall in the foot-hills, about forty feet above the bed of the arroyo, or gulch.
The rough felsite blocks of which they were composed were surprisingly large, considering the diminutive size of the cabins.
We found, besides, a few cores of felsiteand some shapeless flakes and several fragments of large metates.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "felsite" in a variety of sentences. We hope that you will now be able to make sentences using this word.
