59 



of an inch in diameter. Their structure is concentric as the 

 rule, and radiate as the rare exception. Each aggregate (except 

 the smallest, which are usually homogeneous to the eye) consists 

 of a layer of white material enclosed concentrically between a 

 dark-colored centre or nucleus and a thin layer of greenish- 

 white, which comes between the white zone and the external 

 quartzose material. In the unweathered state these different 

 layers have a uniform hardness about equal to orthoclase ; but, 

 when weathered, the dark nucleus becomes of a dull-green color 

 and yields readily to the knife ; and both it and the white band 

 yield the more readily the more the rock is weathered. Where 

 the almond masses are large it can be plainly seen (and proba- 

 bly all would show it under the microscope) that in each of 

 them the general structure of the whole rock is reproduced on 

 a smaller scale, each nodule containing a number of smaller 

 masses of the same form and structure as itself. These smaller 

 or secondary aggregates also have their major axes parallel. 

 They are most conspicuous in the green nucleus. This rock is 

 susceptible of a tolerable polish. 



The question naturally arises as to whether the structure of the 

 "toad stone" is original and general, or due to local alteration. 

 I incline to the former view, although evidence supporting the 

 latter is not wanting. This rock forms the south side and 

 summit of a small hill rising from the tide-water marshes of 

 Mud Creek, and is met on the north by a petrosilex breccia. 

 The junction of the two rocks is marked by a trap dyke, 

 several feet wide, which dips steeply to the north. Now it is. 

 in the immediate vicinity of this dyke that the rock in question 

 possesses the banded structure described on the preceding page ; 

 beyond six inches or a foot from the eruptive mass the banding 

 appears to be wholly wanting. The bands are exactly parallel 

 with the dyke. These facts might be considered as proving a 

 causal connection between the dyke and the pseud-oolitic struc- 

 ture of the adjoining rock. But I think this is illusory ; for in 

 other parts of the petrosilex belt, on Kent's Island, and to 

 the westward, there is evidence showing that this peculiar 



