
ROCKS 75 
structure which is so frequently present in crystalline igneous 
rocks. 
Metamorphic rocks differ much as regards texture and 
structure. They are not all equally crystalline and foliated. 
Some, indeed, show only faint traces either of crystalline or 
schistose structure; others, again, although markedly crystal- 
le are not foliated. But. no foliated rock is devoid of 
crystalline texture. There can be no doubt that many schists 
originally existed as mechanical sediments, and that their 
present constitution is the result of subsequent changes. 
This is proved by the simple fact that all gradations can be 
traced from unaltered sedimentary rocks into rocks which 
become more and more markedly crystalline and foliated as 
they are followed in some particular direction. Again, 
characteristic clastic rocks, such as conglomerate, are not 
infrequently met with intercalated between beds of mica- 
schist or other foliated rock—an arrangement obviously 
suggestive of the original sedimentary character of the latter, 
Further, the occasional occurrence of fossils in crystalline 
schists leaves no room to doubt that such schists are simply 
more or less altered or metamorphosed sedimentary rocks. 
In other cases, however, it can be proved that certain 
foliated rocks are not originally of sedimentary origin. Again 
and again we encounter massive igneous crystalline rocks, 
such as granite, diorite, gabbro, etc, passing outwards by 
insensible gradations into well-marked foliated rocks. In 
such cases we are compelled to conclude that the foliated 
structure has been superinduced. In short, all truly schistose 
masses are metamorphic, and originally existed either as 
igneous or as derivative rocks.* 
Quartzose Rocks.—Amongst these are included several 
rocks, the clastic structure and sedimentary character of 
which are more or less conspicuous. Schzstose Conglomerate + 
is an aggregate of waterworn stones in a crystalline schistose 
* It must be noted, however, that a mass of granite sometimes 
exhibits along its margin a kind of fluxion foliation which is original, 
and not the result of metamorphism. See Chapter XIII. 
t The stones of such conglomerates are often distorted. They may 
be crushed and drawn out into mere lenticles along the planes of 
foliation, as shown in Plate XXI. 3, 
