Modern Igneous Rocks and the Primary Formations. 109 
In the remarks which have been made, I have no where deni- 
ed that gneiss and mica slate may not sometimes be metamorphic, 
but have endeavored to show merely that their stratiform struc- 
ture is no evidence of it. There is reason to believe that some 
of them are altered sedimentary deposits, and to these we may 
add, with as much reason, some granites. 
It has been shown that the foliated structure of mica slate is 
the result of crystallization, whether metamorphic in its origin or 
not. If then certain clayey deposits are so constituted as to form, 
when heated, a large amount of mica, and give rise to mica slate— 
others to form less and produce a gneiss—may there not have 
been other deposits which should have assumed under the trans- 
forming heat the irregular structure of granite ? But without lay- 
ing much stress on this kind of reasoning, let us appeal to modern 
igneous formations for analogies. qin 
In basaltic, porphyritic, trachytic and recent volcanic regions, 
there often occur deposits of argillaceous sandrocks of great extent, 
which have been derived from these igneous formations. The 
basaltic sandrock—called wacke or tufa—frequently so resembles 
basalt in structure and. appearance, that the observer hesitates long 
before he decides upon its nature, and is not fully satisfied, till he 
can discern in some part of the formation, an imbedded pebble to 
assure him of its derivative origin. The only peculiarity it pre- 
sents is a more earthy texture, but this belongs to some true ba- 
salts. I have met with such a rock in Oregon. » The Andes are 
full of similar deposits, both of basaltic, greenstone and porphyri- 
tic origin, and often the closest examination is required to distin- 
guish these sedimentary formations. They consist solely of earth 
or sand, of basaltic or porphyritic origin, which has been rehard- 
ened through volcanic action, and thus made to resume the com- 
pactness that belonged to the parent rock. Much of the so-called 
porphyry of the Andes is a porphyry sandrock, or a sedimentary 
rock of porphyry origin. Jt is as hard and firm in its texture 
as true porphyry, a trachytic variety of which it much resem- 
bles; moreover small crystals of feldspar are thickly disseminated 
through it, and aid in the deception. Were it not that an occa- 
sional pebble may be detected on the weathered surface, no one 
could doubt its being actually an igneous rock. / 
If sedimentary rocks of porphyry and basaltic origin may be so 
remodeled or rehardened by heat, as to be scarcely distinguishable 
