The Geology of Hingham. 7 



a period of 10,000,000 years, and we have a total of 31,845,000 

 years since the globe was separated from the sun in a gaseous 

 condition, and of but 1:3,815,000 years since the first incrusta- 

 tion of its surface. 



Another method of determining the age has been to base esti- 

 mates upon the internal heat of the globe and the rate of cooling. 

 Sir William Thomson thus concluded that about 80,000,000 

 years must have elapsed for the globe to cool to its present con- 

 dition, dating from the first incrustation upon its surface. 



Another method has been to base calculations upon the geo- 

 logical changes that have been going on during comparatively 

 recent times, b}' which sedimentary deposits have been formed 

 at a known rate of thickness within certain periods. Dr. Croll 

 estimates in this way that not less than 60,000,000 years must 

 have elapsed, and probably much more since sedimentation began. 

 Another investigator, Dr. Haughton, on the same basis extends 

 the time to more than 200,000,000 years. 



It is unnecessary to add more on this point. It is sufficient to 

 state that no man capable of forming a judgment, and who has 

 duly investigated the question, has been able to come to any other 

 conclusion than that our good mother the earth has been revolv- 

 ing in her orbit, since incrustation and the commencement of 

 sedimentation, for millions of years, and whether these be num- 

 bered by tens or hundreds can be but of little moment, when the 

 feast mentioned is more than long enough to appall the mind in 

 its contemplation. 



It is however desirable, in view of a better understanding of 

 what may follow relative to different periods in the earth's history, 

 to give a table showing the estimated duration of each, assuming 

 the whole length of time since incrustation to be 80,000,000 

 years, as calculated by Sir William Thomson. Of course, if it 

 should be assumed that the whole period since incrustation was 

 more or less than 80,000,000 years, the time estimated for each 

 period would be proportionately lengthened or shortened. The 

 time ratios of the several periods have been determined by Pro- 

 fessor Dana from the relative thickness of the rocky sediments, 

 and of the probable time required for their deposit, and though 

 estimates thus based must necessarily be imperfect," yet by them 

 we can approximate somewhat nearer to the truth than in any 

 other way. The presentation will be useful in impressing on the 

 mind of the reader the remote antiquity of the rocks of Hingham ; 

 for if, as generally claimed, the greater portion of them had their 

 origin in Archaean Time, basing their age on Sir William's esti- 

 mate of the ao;e of the world, tbev must have been formed more 

 than 30,000,000 years ago. The table is abbreviated from one 

 presented in the very valuable work of Alexander Winchell, 

 LL. D., Professor of Geology and Palaeontology in the University 

 of Michigan, called " World Life, or Comparative Geology." 



