122 Igneous Origin of some Trap Rocks. 
formations of rocks are always interesting to the geologist; they often 
exhibit evidence of change as they approach each other, and indi- 
cate either that different causes were in operation, or that there was 
a different state of the same causes. The more widely the rocks 
differ in their nature, the more strikingly is this fact exhibited. 
But, it is time to give the notice of Rocky Hill, to which the above 
remarks are introductory. 
This is a ridge of trap rock, a subordinate member of the great 
trap ranges of Connecticut and Massachusetts, and like most of them, 
it runs north east and south west. It lies, about three miles nearly 
S.S. W. from the city of Hartford, to which it has for a century, sup- 
plied a large part of the stone used there in architecture. This has 
occasioned an extensive and deep excavation, and the geologist 
owes to the activity and wants of a populous region, the interest- 
ing exhibition of which I am now to give a statement. 
is trap ridge, like all the trap of New England,* reposes upon @ 
sedimentary rock ; a variety of sandstone, but here colored, evident- 
ly by oxide of tre of a deep brownish red, and so charged with 
clay, that it is with great propriety called an argillaceous sandstone. 
Although the trap rock is constantly carried away to repair the roads 
and*to be used in Hartford, in building ; the argillaceous sandstone 
which lies under it, appears to have been the great object for which 
the quarry has been worked. The trap ridge is, on the eastern eXx- 
posure, almost covered with soil. As you come from Hartford on 
ithe old Farmington road, you gently rise the acclivity of a hill for 
perhaps three or four hundred yards, and as you reach its summit, 
you discover the trap ridge, breaking through the soil, on your right 
and left, and stretching away north east and south west like a line of 
fortification. This allusion appears still more appropriate, when you 
discover on coming to the edge of the parapet, that the vast rampart is 
faced with a deep ditch, just such an one as defence would require, 
were the ridge covered by cannon and bristling with bayonets. This 
ditch i is the excavation which, (as its object was peaceful,) it has ta- 
ken a century to make. It is cut, through both the incumbent trap 
and the subjacent sandstone, so that their junction is perfectly exhi- 
bited through nearly a mile in length, and with scarcely an interrup- 
tion. The trap is cut entirely through, and is exposed, from the 
weathered surface, down to its junction with the sandstone ; and this 
~* And like most of that which I have seed in the south of Scotland. 
