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 _wacké_? 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_? 57. What are the great geological agents of change? 58. What is meant by _weathering_? How are rocks affected at the surface in tropical countries? What chemical effect has the atmosphere on calcareous rocks? How is soil formed? How are sand dunes formed? Mention some effects of the transporting power of the atmosphere. 59. Mention some of the chemical effects of interstitial water. What is the origin of _travertine_ or _calcareous tufa_? 60. How have _stalactites_ and _stalagmites_ been formed? Give some instances of the solvent power of springs. 61. How are caves in limestone formed? Describe some of the appearances of a country composed of calcareous rocks. Describe briefly how a river erodes its channel. 62. Describe the geological action of rain. 63. What do chemical analyses of river-water prove? Give an example. What are pot-holes? Give an example of the erosive power of running water. What amount of mud is carried in suspension by the Mississippi, and discharged annually into the sea? What estimate has been formed of the total amount of mineral matter annually transported by that river? 64. What is _alluvium_? How is it formed? and mention some examples of its occurrence. 65. How is sediment deposited by a river in a lake? 66. What is the difference between lacustrine and fluvio-marine deposits? What is a _delta_? 67. Describe the geological action of frost. Describe the geological action of river-ice.