ROCKS. 77 



and their union with the massive rock is 

 to be seen on Mount Holyoke, in Massa- 

 chusetts, in Titan's Piazza, so called be- 

 cause there is a group of columns of 

 greenstone pointing downward from the 

 overhanging rock above. 



The manner in which rocks of this 

 family are formed will be considered in 

 another part of this book. 



154. Trappean Rocks in this Country. 

 Beginning at the north, there is a belt of trap extending 

 130 miles along the Bay of Fundy, where the violence 

 of the waves has exposed to view magnificent groups of 

 columns three and four hundred feet in height. In the 

 neighborhood of Boston, at Nahant, Lynn, etc., there are 

 ridges, some of them rising to the height of 500 feet. 

 A range beginning at East and West Rock, New Ha- 

 ven, extends up the valley of the Connecticut almost to 

 Vermont, including Mounts Tom and Holyoke, which 

 rise to a height of over a thousand feet. The noted 

 Palisades, on the Hudson, are greenstone. Three ridges 

 of trap extend through the State of New-Jersey, and 

 trap rocks are found in beds and elevations as far south 

 as North Carolina. But it is west of the Rocky Mount- 

 ains that trap most abounds and reaches the highest ele- 

 vations. Columbia River has on each side of it mount- 

 ains of trap, in some cases even a thousand feet in height. 

 The appearance of the columns is seen in Fig. 30, p. 78. 



155. Lavas. The materials thrown out from volca- 

 noes become, as they solidify, what are called lavas. The 

 various kinds differ in structure and composition, and, 

 of course, in color. There are two classes of lavas the 

 light colored, in which feldspar is the chief constituent, 

 and the dark colored, or basaltic, which are grayish- 

 blue, even to black. The structure of lavas depends 

 upon circumstances. That which cools under pressure, 

 and while shut in from the atmosphere, is compact ; but 



