TEACHERS' SCHOOL OF SCIENCE. 645 



of action, the cutting backward by the cascade action, the cutting 

 downward of torrent action, and the more quiet transportation and 

 final deposition of the streams passing through the lower levels and 

 approaching the sea. From the sides of Hoosac and Greylock the 

 surface of the IV^ssachusetts plateau is seen, with its dissection by 

 the Berkshire and Deerfield Valleys, illustrating the broad effects 

 of erosion over the surface of the continent. 



Passing next to a discussion of the disposition of the material 

 that is derived by erosion from the land, a lecture upon the sorting 

 action of water is given, and the resultant beds of gravel, sand, and 

 clay are studied in a section cut by the Fitchburg Railroad through 

 the sand plateau at Lake Walden, in Concord. 



The next step is to study these products of deposition in their 

 consolidated forms. . At Parker Hill, Roxbury, a large quarry fur- 

 nishes opportunity for the study of conglomerate, special attention 

 being paid to the means of determination of stratification in a nearly 

 homogeneous, coarse material. Here also is a large section in. a 

 drumlin left in a nearly vertical face by excavation about twenty 

 years ago, and now illustrating finely the action of rain during the 

 years. This forms an instructive contrast with the marine erosion 

 of Great Head, Winthrop. Any one of the numerous slate quar- 

 ries at Somerville serves the purpose of studying stratification in a 

 fine, homogeneous material. In each of these three last-named 

 places the various phenomena of stratified rocks are studied, such as 

 unconformity, cross-bedding, ripple-marks, strike, and dip, but at- 

 tention is confined more especially to the original structures, sub- 

 sequent structures being left for later lessons. 



Eruptive rocks are then taken up and studied in respect to their 

 origin and original structures. The quarries near Winter Hill, in 

 Somer^nlle, furnish an admirable opportunity to study dikes. Here 

 a small hill of slate is intersected by three series of dikes of differ- 

 ent character and intersecting each other at various angles, enabling 

 a determination of their relative ages. An intrusive bed, now sepa- 

 rated from its parent dike by erosion, affords the means of compar- 

 ing the characteristics of the two forms and of tracing out the rela- 

 tion between them. The inclined positions of the dike and bed 

 and the numerous quarries furnish several sections in varying rela- 

 tions to the two. The various dikes and the inclined position of the 

 inclosing slate give an excellent chance for the first instruction in 

 the making of geological maps and sections. ISTotes are taken for 

 this purpose, and both maps and sections are constructed and handed 

 in at a later date. 



At Marblehead I*^eck various other eruptive structures, such as 

 flow structure, ancient ash-beds, etc., are seen in the felsite, of which 



