Consider the following statements concerning engineering geology: 
1) Dams on sedimentary impervious strata dipping upstream will be unsafe 
2) In general, igneous rocks are hard and are suitable for construction of large hydraulic structures. 
3) Construction of a major dam must be preceded by thorough geotechnical investigations. 
4) Construction of dams is never indicated in any active zone of seismicity. 
Which of the above statements are correct?