oo4 F, RUTLEY ON COMMUNITY OF STRUCTURE 
These are only a few examples, and a comparison of them does 
not certainly seem to promise any useful results. 
The occurrence of fragments of crystals with sharp angles may 
be due either to the fracture and comparatively slight displacement 
of crystals developed in a rock, such as the fractured and faulted 
-erystals sometimes met with in eruptive rocks of various kinds, 
especially in lavas; or to the fracture and wide dispersion of the 
resulting fragments ef crystals, such as one meets with in volcanic 
ejectamenta and in sedimentary rocks. ‘The one case differs from 
the other only in the fact that, in the case of imbedded crystals, the 
dispersion of the fragments could not take place to any great extent ; 
while in the other instance the crystals have been entirely or par- 
tially isolated prior to fracture, and the fragments were consequently 
free and capable of performing long journeys. Such fragments 
would naturally lose more of their angularity in proportion to the 
distance over which they travelled, unless transported under condi- 
tions in which attrition was impossible. 
That microcrystalline felsitic matter and eryptocrystalline felsitic 
matter, or ordinary felstones, felspathic sandstones and grits, arkose, 
granulite, and devitrified hyaline rhyolites, are closely allied is a 
point which I shall now endeavour to demonstrate. 
The terms microcrystalline and cryptocrystalline, as used by Prof. 
Rosenbusch, are here purposely employed to designate the condi- 
tions of felsitic matter known as felstone, because they have, or are 
assumed to have, a more or less definite mineral constitution, as 
implied by the term ortho-felsite of Dr. Sterry Hunt, consisting of 
crystalline grains of felspar (typically orthoclase) and quartz. These 
grains do not always show definite boundaries, but often appear 
under the microscope, in. polarized light, to shade off one into 
another, representing what Zirkel describes as ‘ unindividualized 
granules.” 
When the grains are individualized it then becomes a matter of 
considerable difficulty to distinguish between such felstones and 
certain granitoid rocks such as haplite and perhaps granulite, and 
also between such felstones and certain sedimentary rocks in which 
little or no perceptible cementing matter occurs, as in some examples. 
of felspathic grit, sandstone, and arkose ; and I believe that to this 
similarity is due much of the confusion in the statements concerning 
the origin of some of our bedded felstones, some observers main- 
taining that they are contemporaneous sediments in which erys- 
tallization from aqueous solution has supervened, while others 
contend that they are true eruptive rocks which have crystallized 
on cooling. Be their origin, however, what it may, the point which 
I now wish to establish is their community in mineral constitution 
and their approximate or, possibly, in some cases absolute community 
of structure. 
Felsitic matter, however, as Zirkel observes, must not always be 
confounded with the idea of felstone. Felsitic matter is a very 
comprehensive term, and, in spite of all that has been written about 
