50. What is a _fault_? What is _hade_? How are the strata affected on
either side of a fault? What is the appearance called _slickensides_?
Under what circumstances should we term a fault a _downthrow_? and
when should we term it an _upcast_? How is the approximate age of a
fault sometimes shewn?
51. What are metamorphic rocks, and what is their general appearance?
In what districts of the British Islands are they most abundantly
developed? What are some of the appearances relied upon for
distinguishing metamorphic from igneous granite?
52. How do igneous rocks occur? Define what is meant by
_contemporaneous_ and _subsequent_ or _intrusive_ igneous rocks. How
does a contemporaneous igneous rock affect the beds upon which it
rests? What is the character of the bed overlying a contemporaneous
rock? What is the general structure of a contemporaneous igneous rock?
What is meant by _vesicular structure_? What is the general texture of
a contemporaneous igneous rock? What is the nature of the jointing in
igneous rocks? What is _wacke_?
53. What is the nature of the beds of _breccia_, _conglomerate_,
_ash_, and _tuff_, with which contemporaneous igneous rocks are often
associated? What is a _neck_ of _volcanic agglomerate_? How are the
strata affected at their junction with a 'neck'?
54. How do intrusive igneous rocks occur? How do intrusive _sheets_
occur? What effect have they produced upon the strata above and below
them? What is a _dyke_? What relation do they occasionally bear to
_sheets_ of igneous rock? What is a _neck_ of intrusive igneous rock,
and how have the strata surrounding it been affected?
55. Mention some of the contrasts between _intrusive_ and
contemporaneous igneous rocks. What alteration is produced upon coal
with which an intrusive sheet has come in contact?
56. What are _mineral veins_? What is the nature of the quartz veins
in granite? How are the minerals usually arranged in the great
metalliferous veins? What is a _pipe-vein_?