IN ROCKS OF DISSIMILAR ORIGIN. 305 
it, a very ill-defined term for matter possessing very ill-defined 
structure. Itis commonly stated that in its most typical develop- 
ment it is almost totally amorphous, frequently exhibiting little or 
no double refraction. Now I wish this point to be specially borne 
in mind, because we often find that more or less clear and translucent 
amorphous matter, glass in fact, both artificial and natural (as 
obsidian, pitchstone, &c.), has undergone change of such a nature 
that what was glass is no longer glass, what was once amorphous 
is now crystalline in structure, and that this change has resulted in 
the production of felsitic matter. 
Many of these hyaline rhyolites have approximately the chemical 
composition of felspars; and we find not merely such amorphous 
rocks, but at times we also meet with individual crystals of felspar 
converted into a similar felsitic substance. 
It is common to find that in proportion to the age of such rhyolitic 
rocks so is the change which they have undergone, and, in extreme 
phases of devitrification, it often becomes most difficult to speak 
with any thing like certainty about their origin or to distinguish 
them from sedimentary rocks, such as felspathic grits and sandstones. 
Evidence, indeed, is not wanting, especially in North Wales and 
in the Lake-district, to show that rocks which are commonly re- 
garded as volcanic ashes, and which certainly have a fragmentary 
constitution, pass by almost insensible degrees into true felstones or 
hilleflintas. 
Here, then, we find that we have at times scarcely any means 
of distinguishing between lava-streams which have represented the 
most extreme phase of igneous fusion, clastic rocks, possibly repre- 
senting volcanic ejectamenta, and detrital rocks which have been 
deposited as sediments in water. The rocks of all these classes 
have once formed superficial layers and may consequently occur 
interbedded with stratified rocks. 
It may at once be said, ‘“‘ Look for alteration of beds subjacent to 
lava-flows!” A good and wise precaution! but alteration is not 
always to be found. Where this test fails, how shall we manage 
to assign a definite origin to such rocks with any certainty? This 
is best answered by an examination of recent vitreous rocks, which 
very frequently present distinctive characters. One of the most 
marked is the phenomenon of fluxion-structure ; yet even this is to 
a certain extent simulated in detrital deposits. 
It is not, for instance, uncommon to find large stones imbedded 
in laminated sediments ; and where this is the case the laminve which 
overlie the stone, and often those below it, are deflected from the 
general planes of lamination and sweep round the unyielding mass 
in curves which approximately correspond with its outline. We 
find the same thing reproduced on a small, often on a microscopic, 
scale in schistose volcanic ashes and tuffs; and the appearances thus 
produced are frequently very suggestive of fluxion-structure and are 
extremely deceptive to those who haye had but little experience in 
the microscopic diagnosis of rocks. 
Let us now consider tho characters by which we may or may 
not safely distinguish volcanic ejectamenta from other rocks, 
