12 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



lized and hardened from the pressure and heat of the depths. In 

 this way limy sediments of the sea, after deep burial and sub- 

 jection to mountain-making forces, become crystallized to 

 marbles. Muds, first hardened into shales, are finally trans- 

 formed into sparkling mica schists. Sandstones pass into lustrous 

 quartzites. Granites are mashed into banded rocks known as 

 gneisses. All these crystallized and hardened forms are classified 

 as metamorphic rocks. They are the foundation whose broad 

 exposure at the surface testifies to the destruction of ancient 

 mountain ranges. Their resistance prolongs the erosion cycles 

 which upon eacli renewed uplift begin their re-destruction. 

 Let them be planed down to the level of the sea and then uplifted. 

 The upwarped peneplain will endure for a time, largely undes- 

 troyed, existing as a plateau trenched by narrow valleys even 

 after the softer rocks have been again eroded to another low-lying 

 plain. Or let submergence occur. The rivers and the sea will 

 then lay down layers of sand and mud across the level floor of 

 contorted and eroded mountain structures. The surface of the 

 ancient land is now a surface of unconformity, and the latter 

 as a record of an erosion cycle has become a part of the geologic 

 story, but the record is concealed unless new forces warp or fold 

 the rocks and again subject them to erosion. 



The Measure of Geologic Time. Man measures his life by 

 a few scores of years, but the years of the earth are measured by 

 many millions, an abyss of time so vast in comparison that the 

 mind cannot fathom it save by the use of analogy. Let a year 

 be represented by a foot; the average length of human life is 

 measured then by the breadth of a dwelling house, 'and human 

 history is limited approximately to a mile; but the duration of 

 geologic time is measured in terms of the circumference of the 

 globe. 



The length of geologic ages cannot be stated accurately in 

 years, but the rather conservative estimates of J. D. Dana are 

 given in the annexed table of geologic time. Certain lines of 

 evidence suggest that the geologic ages may be many times 

 longer, but no reliable estimate yields a lesser duration than that 

 given long since by Dana. The ratios of the relative duration 

 of the eras are presumably more reliable than the estimates of the 

 lengths in years. It is seen that each preceding era of the last 



