Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1045

 

Summary of Inferences.

From the details, somewhat lengthy I fear, given above, we are, I think, entitled to draw the following conclusions:—

(1) That the serpentine of the Lizard was originally an intrusive igneous rock.

(2) That its intrusion was posterior to the metamorphism of the hornblende schist, the fragments of that rock included in it not differing materially from the main mass, though, of course, a little more altered.

(3) That on the western coast the serpentine has been broken through by several granite dykes.

(4) That on the eastern coast it has been broken through by the following igneous rocks:—

(a) Gabbros, which at Coverack Cove are certainly of two very distinct dates; and the later are most probably of the age of the great gabbro mass of Crousa Down. Similarity of mineral character would lead me to regard all the east-coast outbreaks as far as the Balk as of the same date, viz. that of the Crousa-Down massif. (b) Certain dark trap dykes found, like the gabbros, only on the east coast, having much the same range, and the latest of all in date.

(5) That the gabbros appear liable to three forms of mineral change.

(a) The gradual conversion of their felspar into a microcrystalline saussuritic mineral.

(b) The conversion of their diallage into hornblende by pseudomorphism, or rather by a recrystallization, not generally by paramorphism, and that in certain cases some olivine disappears in this process.

(c) The conversion, more or less complete, of the olivine into serpentine, in which case the diallage appears little changed.

(6) That these trap dykes were very probably once all dolerites or basalts, and that the hornblende, which undoubtedly characterizes many of them, is a secondary product due, as above, to metamorphism of the original pyroxenic constituent.

(7) That the metamorphism of the serpentine was probably complete before the intrusion of any of the above rocks.

(8) That the serpentinous aspect of a rock is often rather illusory, being due to the presence of an extremely small proportion of that mineral; hence that statements about the conversion of ordinary pyroxenic or hornblendic rocks into serpentine require confirmation from microscopic examination.

 

Microscopic Examination of the Serpentine.

With the view of ascertaining, if possible, the original character of the rock which now constitutes the serpentine of the Lizard, I have