Page:The Geologist, volume 5.djvu/71

It is well known that prehnite is a common mineral in some parts of this colony. It occurs in the dioritic or syenitic dykes of the Dicynodon strata, and is evidently a product of the re-arrangement of the minerals of these dykes during their decomposition. The prehnite is found in laminæ between the decomposing masses of the dykes and on their faces as digitate concretions. Unlike M. Daubrée's zeolites it has not required heat for its formation, but, like them, it is forming at this hour by aqueous action. I have almost as little doubt that dykes and other masses of granite are forming in the same way from the re-arrangements of the constituent minerals of the gneiss. I find it easier to feel the truth of this on the spot than to convey it to others; still I will endeavour to give some reason for my faith. One mass of gneiss, near Henkrees, in which a vein of granite, terminated by an expansion about a foot square, proceeded along the gneiss. It was quite evident that there was no intrusion, no connection with any mass below. Spindle-shaped masses of granite placed between beds of felspathic, micaceous, and other rocks, were numerous and could often be entirely removed by a hammer, showing they had no connexion with any subterranean mass. Spherical lumps of granite or syenite were frequently quite isolated in the schists of the mines. I have stated before that beds of quartzite appeared to be intercalated by a filtration from above; some of these could be traced down till they became mixed with mica and then with felspar, and not very much deeper assumed the character of the gneiss of the country. Other masses, which seemed to be intermediate between these, toward the spindle-shaped granite lumps above, were composed principally of quartz, with a few grains of mica and felspar, and occasionally garnets: these were surrounded by mica-schist in bent-up strata; yet the whole mass, perhaps twenty pounds or less in weight, could be removed. These circumstances seemed to me to prove clearly that if felspathic rocks of any kind are the products of metamorphic action, then are granite, syenite, etc., traceable to a like origin; that if, as I think, I have clear evidence, in the Maitland Mines and other places of this neighbourhood, palæozoic rocks are convertible into micaceous, chloritic, and felspathic schists, without evidence of eruptive agencies, then are the so-called igneous rocks equally so. I think it will scarcely be disputed by any who admit the re-arrangement of felspathic rocks I have contended for, that causes by which such re-arrangement was made, may have effected the original conversion of sedimentary into crystalline rocks. We have in this province a rock which has been pronounced decidedly igneous by the highest European authorities; it is the claystone-porphyry of Bain. Yet its position among the other rocks is, in many respects, just that of the quartzite; and, like the latter rock, its individual beds and its minor masses are conformable with the stratified rocks, while in ranges many miles in length, it crosses the strike of the strata, generally at an angle of about 30°. This fact was first demonstrated to me by Mr. Pinchin, a gentleman who has made many interesting observations on the geology of this country.