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Tom Swift and His Big Tunnel, or, the Hidden City of the Andes by Victor [pseud.] Appleton
page 23 of 219 (10%)
"A snag in the shape of extra hard rock," replied the
tunnel contractor. "Briefly, Paleozoic rocks make up the
eastern part of the Andean Mountains in Peru, while the
western range is formed of Mesozoic beds, volcanic ashes and
lava of comparatively recent date. Near the coast the lower
hills are composed of crystalline rocks, syenite and
granite, with, here and there, a strata of sandstone or
limestone. These are, undoubtedly, relics of the lower
Cretaceous age, and we, or rather, my brother, states that
he has found them covered with marine Tertiary deposits.

"Now this Mesozoic band varies greatly. Porphyritic tuffs
and massive limestone compose the western chain of the Andes
above Lima, while in the Oroya Valley we find carbonaceous
sandstones. Some of the tuffs may be of the Jurassic age,
though the Cretaceous period is also largely represented.

"Now while these different masses of rock formation offer
hard enough problems to the tunnel digger, still we are more
or less prepared to meet them, and we figured on a certain
percentage of them. Up to the present time we have met with
just about what we expected, but what we did not expect was
something we came upon when the tunnel had been driven three
miles into the mountain."

"What did you find?" asked Tom, who knew enough about
geology to understand the terms used. Mr. Damon did not,
however, and when Mr. Titus rolled off some of the technical
words, the drug investor softly murmured such expressions as

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