20. How do _stalactites_ and _stalagmites_ occur? What is _siliceous sinter_, and how does it occur? How does _rock-salt_ occur? 21. Mention some of the varieties of limestone. What is _cornstone_? What is the composition of _dolomite_? 22. Name some of the varieties of coal. 23. What is _quartzite_? 24. Describe _clay-slate_. 25. Mention some altered limestones. 26. What are _schists_? Name and give the mineralogical composition of three schists. 27. What is the general character of metamorphic rocks? 28. How would you classify granite? 29. What is the mineralogical composition of _syenite_ and _diorite_? 30. How do we distinguish the two groups into which igneous rocks are subdivided? What is meant by the terms _amygdaloidal_ and _porphyritic_? 31. Name some rocks that belong to the acidic group. What is _quartz-porphyry_? 32. Give examples of augitic igneous rocks. Name a hornblendic igneous rock. 33. What are fragmental igneous rocks? What is the difference between _trappean breccia_ and _trappean conglomerate_? 34. What is meant by the terms _stratum_, _strata_, and _stratified_? What is the difference between _lamination_ and _bedding_? What is a section? 35. What is _false bedding_? 36. Briefly describe the general appearance of _mud-cracks_ and _rain-prints_, and say how these have been formed. 37. What is meant by a _succession of strata_? 38. Which kinds of stratified rocks generally have the greatest extension? 39. How do beds terminate? 40. How may planes of bedding sometimes indicate a break in the succession of strata? 41. What is the nature of _joints_? What are _master-joints_, and what is their probable cause? 42. What is _cleavage_, and what is its effect upon the bedding of rocks? 43. What is _foliation_? 44. Give examples of concretionary rocks. What is the nature of chert and flint nodules? 45. Define the terms _dip_ and _strike_. What is the _crop_ of a bed? What are _anticlines_ and _synclines_? 46. What is meant by an _inversion of strata_? 47. How does contemporaneous erosion indicate a pause in the deposition of a series of strata? 48. What is meant by _unconformability_? How does unconformability prove a lapse of time between the accumulation of the underlying and overlying strata? 49. What is _overlap_? 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?