Sentence:  In several important cases, it is difficult to determine to what degree the metamorphosis has extended, since it remains uncertain whether the form and structural relations of the mountain-mass are to be considered as remains of the <WSD>neptunian</WSD> sedimentary formation, or as a product of the metamorphosis itself; for the original form of the <WSD>neptunian</WSD>, as well as of the volcanic sedimentary rocks have often become so changed by erosion, and so much alike in both classes of rocks, that this distinctive character is wholly lost; [...] 
Meaning of the word marked with <WSD> tags:  Of, pertaining to, or supporting (rocks were formed from the crystallisation of minerals in the early Earth's oceans).