159 



this rock forms a high precipitous ledge, of which the culminating 

 point is Sunset Rock. The buildings of the Theological Seminary- 

 appear to be on the boundary between the coarse granite and the 

 gneiss to the west. The most easterlj^ outcrop of the coarse rock is 

 near Simeon's, west of H. Gray's in the Holt District. The length of 

 this granite is a little more than four miles ; its greatest width 440 

 rods. The crystals of mica and feldspar are sometimes six inches 

 broad, and are universally very large. 



The mica schist is a fine-grained rock, often running into quartzite, 

 and occupies the low land about the Merrimac Kiver. It appears to 

 occupy all the area 150 rods south of the river, from near the mouth 

 of Fish Brook to Boxford. Its dip is commonly 30°-40° north-west- 

 erly, but it is 53° N. at Daniel Butler's house. It therefore appears to 

 lie unconformably upon the gneiss, but farther observations are 

 needed to confirm this inference. 



There is reason to believe that the gneiss and hornblende schist 

 are of Eozoic ages, perhaps as old as the Lavirentian, and hence that 

 Audover Hill has been dry land ever since the oldest period, except 

 when covered by the waters of the later drift. The evidence consists 

 chiefl}^ in the fact that pebbles of the syenite, which is newer than the 

 schist, occur in the Paradoxides slates near Boston, along with red 

 jasper, green porphyry, and other rocks associated with the syenite. 

 These slates form the lowest member of the Paleozoic series ; hence 

 the rocks from which the pebbles were derived are older than the Si- 

 lurian, and must be Eozoic. Lithologically they resemble the Lauren- 

 tian gneiss and syenite, in the typical localities. The coarse granite 

 being intrusive must be newer than the gneiss. The fine mica schist 

 was at one time suspected to be Carboniferous, on account of the oc- 

 currence in it of decomposing cylindrical masses resembling the stems 

 of trees. It is connected with the plumbaginous rock near "Worcester. 



Nearly the whole of the surface of Andover is covered by the 

 coarse unmodified glacial drift. Striae are occasionally seen on the 

 ledges, as near Carmel Hill, with the direction (by compass) of north 

 and south. Bowlders of porphyritic granite and other rocks peculiar 

 to New Hampshire may be found upon the highest land. Holt's or 

 Prospect Hill, and the round hills to the north-east appear to be com- 

 posed of the coarse drift, and may be the remnant of a moraine of the 

 continental glacier. 



Lying upon the older drift are the curious "Indian ridges "and 

 "moraine terraces," both of which appear to have been deposited 

 from water, as a cut in them invariably shows the presence of strata. 

 According to a map by Mr. Alonzo Gray in the Transactions of the 

 American Association of Geologists and Naturalists, the ridges are 



