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



The graphic method has the disadvantage, on the other hand, 

 of requiring the definite expression of detail, where in the nature 

 of the problem a knowledge of detail is more or less absent ; 

 but this defect, inherent in drawings, is seen to be unimportant 

 if the reader follows the evidence on which they are based and 

 uses them for the purpose of visualizing general conclusions. 



The conventional structure sections show neither the land- 

 scape of the background nor the clouds above, but for the present 

 purpose these may effectively be added. The atmosphere and its 

 clouds belong to the earth. In wind and rain they play their 

 parts in the geologic drama. Climate is expressed in the present 

 to some extent by cloud forms, and ancient climates are recorded 

 in the crust by the character of the contemporaneous erosion 

 and sedimentation, the work of former sun and frost, of rain 

 and wind, of moving ice and water. Furthermore, each type of 

 cloud has a tendency toward a certain size and elevation, and gives 

 a rude gigantic scale against which may be measured the mountain 

 heights. Observations at the Blue Hill Observatory, for example, 

 showed that the cumulus, or summer day clouds, in summer have 

 their flat bases at an average elevation of 4,900 feet above the land 

 surface, in winter at an elevation of 4,600 feet. Their rounded, 

 tumultuous summits rise to an average of 1,500 feet above their 

 bases. 1 The heights as found in other countries are not markedly 

 different, but the average height increases about 1,400 feet from 

 morning to noon, and from day to day may depart from the mean 

 for the hour of the day within somewhat similar limits. The flat 

 base of the cumulus may be regarded, therefore, as usually 

 ranging from three-quarters of a mile to a mile and a quarter 

 above the surface of the plains. 



The Forces of Geologic Change. Sun and frost, air and 

 rain, slowly cause the rocks to crumble, and with the aid of 

 plants convert them into soil. But the soil creeps down the 

 slopes ; it is partly dissolved by water and partly blown away by 

 wind. Rivulets carry this land waste to rivers. Rivers grind 

 their channels deeper by sweeping along the bottoms the pebbles 

 and the sands. When the rivers have reached their lowest level 

 they lay down their burden by spreading it over lowlands or giv- 



: Clayton, II. H., and Ferguson, S. P., Measurements of Cloud Heights and 

 Velocities. Annals of Astronomical Observatory of Harvard College, Vol. xxx, 

 Part iii, 1892. 



