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structure of porphyritic lava is altogether different from that 
of recent lava from active volcanoes, and tends to show that 
other forces than mere fusion and solidification have been at 
work. The origin of metamorphic rocks also remains still 
unexplained. 
15. They present evident signs of stratification, as if deposited 
from water, and contain fossil remains, and yet possess a more 
or. less crystalline structure, closely resembling that of the 
primary rock. Strange theories have been made to explain this 
double character, and fire and water, electricity and magnetism, 
have been called in to account for them, without an attempt 
being made to explain how they can have produced the result. 
A careful and patient study of these rocks may ultimately lead 
to an understanding of their true nature, but will also without 
doubt impress us with the variety of conditions that have been 
brought about in their formation. The pseudo-morphic minerals 
form in this respect a most interesting study. In them we 
have the form of one mineral, and the constitution of another ; 
showing that since the first formation of the mineral, other 
agents have so altered it, that it is in fact a cast of the original 
crystal in new matter. Probably in all these cases, the element 
which has been substituted for another, has acted in solution 
upon the original crystal, the original constituent being carried 
off in solution, without alteration of the original form of the 
mass. But when we endeavour to trace the origin of these 
solutions which have acted thus locally, and as it were 
capriciously, we find it impossible to do so. 
16. To a certain extent the formation of the stratified rocks 
is less difficult to understand. The disintegration of primary 
rocks, and the gradual elutriation of streams, give an easy 
explanation of the formation of the sand and clay, which form 
the basis of the non-calcareous rocks. An examination of the 
geology of Dartmoor or Cornwall will easily show us the 
decomposition of granite, and the separation of the detritus by 
elutriation into clay and sand, the alkali being carried off in 
solution, and we can almost watch the progress of the process. 
17. If, however, Ave examine the beds deposited in the 
Moreton Hampstead Valley, we shall see that this has gone on 
with very different rapidity at different times. If it were con- 
tinuous, the valleys should be filled by a continuous deposit, 
hut we find alternations of clay and sand, and peat, evidently 
pointing to great changes of the condition of formation. 
18. The consolidation of sands and clay into sandstone and 
