Geology and Mineralogy. 387 



the mountain ranges which may then have existed in the regions 

 of the present Highlands. The draining of the Connecticut 

 estuary occurred at the close of Triassic time. The whole area 

 of Connecticut was reduced to a peneplain in later Mesozoic 

 time. A general elevation of the country in Tertiary time led to 

 the development of the broad features of the present topography. 

 While the action of erosion on the soft rocks of the Central 

 Lowland was able to reduce that part of the state again to a con- 

 dition approaching a peneplain, in the same lapse of time the 

 streams working on the hard rocks of the Highlands could only 

 carve narrow, gorge-like valleys. While the broad features of 

 the topography were shaped in Tertiary time, many minor details, 

 such as drumlins, river terraces, waterfalls, lakes, and harbors, 

 were due to the changes of Quaternary time." 



In regard to the geological history of the Connecticut crystal- 

 lines the junior author states as follows : 



" Fossils are the most readily applicable means of determining 

 the age of rocks, and the only fossils found within the state are 

 those of the Triassic sandstones— chiefly fishes, reptiles and 

 plants. There is, therefore, no formation . within Connecticut 

 older than the Triassic, whose age has been definitely determined 

 by the examination of the rocks themselves. Their position in 

 the time scale can be determined only by comparison with other 

 regions where similar rocks are present in known relations. By 

 combining such comparison with the study of the structure of 

 the rocks themselves, it is found that the crystallines within the 

 state have had a long and complicated geological history, begin- 

 ning before the earliest fossiliferous strata were deposited. 



" Rocks older than the Cambrian exist in Connecticut, but we 

 are ignorant of their origin and exact age. They may have been 

 sediments containing fossils, or they may have been igneous 

 masses. Whether they represent the Archaean or the Algonkian 

 system, or both, is unknown. Their position and structure and 

 texture are so altered by metamorphism that all evidences which 

 might be used in determining their age have been destroyed. 

 There is, however, little warrant for assuming in general that the 

 gneisses and granites in Connecticut represent parts of the ' ori- 

 ginal earth's crust.' The Becket gneiss and certain other forma- 

 tions in Connecticut are believed to be pre-Cambrian, although 

 without conclusive evidence. 



" A study of the Cambrian rocks of North America shows the 

 distribution of land and water to have been very different from 

 the present. There are some facts that suggest that Connecticut 

 was under water, and that a land mass existed to the eastward 

 beyond the present shore line. It has been fairly well demon- 

 strated that a sea or bay of salt water stretched across New Eng- 

 land and up to the St. Lawrence. The extent of this Cambrian 

 sea is unknown ; but part of the marble of the Housatonic valley 

 is believed to have been made from calcareous mud deposited at 

 that time; and the quartzite at Poughquag, just west of the 



