72 New York State Museum, 



body of magma also manifests itself in the rocks in the 

 form of joints which run in various directions dividing 

 igneous rocks into variously shaped blocks and permitting 

 the entrance of air and water to act as agents in weather- 

 ing and decay. Columnar structure is a type of jointing 

 best exemplified in basalts, though it is found in all kinds 

 of igneous rocks and in both extrusive and intrusive oc- 

 currences. The whole rock mass is made up of columns 

 fitted together in a regular manner. Such columns vary 

 from a few inches to several feet in diameter and from 

 a foot to one or two hundred feet in length. In a lava 

 flow or intruded sheet, such as forms the Palisades of 

 the Hudson, the columns are vertical ; in a dike, more or 

 less horizontal. 



Relative age of igneous rocks. This is determined by 

 their relation to the country rock and to each other. An 

 igneous rock is younger than the rock into which it is 

 intruded. In intersecting dikes, the one that cuts across 

 the other is the younger. A surface flow is younger than 

 the rocks over which it flows and, when buried, older 

 than the material deposited above. Igneous rocks also 

 are younger than faults that displace them. The geologic 

 age of igneous rocks can be determined only by the re- 

 lation that they bear to sedimentary rocks whose age is 

 known. 



Metamorphic Rocks 



Metamorphic rocks as already defined are rocks, 

 either igneous or sedimentary, which have undergone 

 certain alterations in mineral composition or texture 

 or both. The chief agents of metamorphism are 

 mechanical movements of the earth's crust and pressure, 

 the effect of heat and the chemical action of liquids 



